Hydro Pump is learned by 96 species by level, 176 by machine and 12 by egg

Effect, in PokéAPI's words

“Inflicts regular damage with no additional effect.”

Who learns it

181 species in all, 7 of 25 games shown

Scarlet and Violet: 132 species

Species	Method	Level
Cloyster	Level up, Machine	1
Politoed	Level up, Machine	1
Marill	Level up, Machine	30
Squirtle	Level up, Machine	33
Popplio	Level up, Machine	36
Mudkip	Level up, Machine	39
Golduck	Level up, Machine	40
Gyarados	Level up, Machine	40
Azumarill	Level up, Machine	40
Poliwag	Level up, Machine	42
Piplup	Level up, Machine	43
Oshawott	Level up, Machine	43
Chinchou	Level up, Machine	44
Wartortle	Level up, Machine	45
Horsea	Level up, Machine	45
Buizel	Level up, Machine	45
Luvdisc	Level up, Machine	46
Poliwhirl	Level up, Machine	48
Tentacool	Level up, Machine	48
Shellder	Level up, Machine	48
Froakie	Level up, Machine	48
Blastoise	Level up, Machine	49
Vaporeon	Level up, Machine	50
Totodile	Level up, Machine	50
Prinplup	Level up, Machine	50
Brionne	Level up, Machine	50
Finizen	Level up, Machine	50
Palafin	Level up, Machine	50
Seadra	Level up, Machine	51
Kingdra	Level up, Machine	51
Milotic	Level up, Machine	52
Dewott	Level up, Machine	53
Lanturn	Level up, Machine	54
Lapras	Level up, Machine	55
Croconaw	Level up, Machine	55
Marshtomp	Level up, Machine	55
Alomomola	Level up, Machine	55
Skrelp	Level up, Machine	55
Frogadier	Level up, Machine	56
Cramorant	Level up, Machine	56
Floatzel	Level up, Machine	57
Tentacruel	Level up, Machine	58
Primarina	Level up, Machine	58
Empoleon	Level up, Machine	59
Dragalge	Level up, Machine	59
Pelipper	Level up, Machine	62
Inteleon	Level up, Machine	62
Swampert	Level up, Machine	63
Samurott	Level up, Machine	63
Keldeo	Level up, Machine	63
Lombre	Level up, Machine	64
Volcanion	Level up, Machine	66
Greninja	Level up, Machine	68
Feraligatr	Level up, Machine	70
Suicune	Level up, Machine	72
Lugia	Level up, Machine	72
Kyogre	Level up, Machine	72
Iron Bundle	Level up, Machine	77
Walking Wake	Level up, Machine	84
Palkia	Level up, Machine	88
Psyduck	Machine
Poliwrath	Machine, Egg
Slowpoke	Machine
Slowbro	Machine
Seel	Machine
Dewgong	Machine
Rhydon	Machine
Dratini	Machine
Dragonair	Machine
Dragonite	Machine
Mew	Machine
Wooper	Machine
Quagsire	Machine
Slowking	Machine
Qwilfish	Machine
Tyranitar	Machine
Ludicolo	Machine, Egg
Wingull	Machine
Surskit	Machine, Egg
Masquerain	Machine
Barboach	Machine
Whiscash	Machine
Corphish	Machine
Crawdaunt	Machine
Bagon	Machine
Shelgon	Machine
Salamence	Machine
Rayquaza	Machine
Shellos	Machine
Gastrodon	Machine
Finneon	Machine
Lumineon	Machine
Rhyperior	Machine
Phione	Machine
Manaphy	Machine
Arceus	Machine
Basculin	Machine
Swanna	Machine
Hydreigon	Machine
Clauncher	Machine
Clawitzer	Machine
Goodra	Machine
Avalugg	Machine
Mareanie	Machine
Toxapex	Machine
Dewpider	Machine
Araquanid	Machine
Bruxish	Machine
Sobble	Machine
Drizzile	Machine
Chewtle	Machine
Drednaw	Machine
Arrokuda	Machine
Barraskewda	Machine
Pincurchin	Machine
Eiscue	Machine
Drakloak	Machine
Dragapult	Machine
Basculegion	Machine
Overqwil	Machine
Quaxly	Machine
Quaxwell	Machine
Quaquaval	Machine
Wiglett	Machine
Wugtrio	Machine
Veluza	Machine
Dondozo	Machine
Tatsugiri	Machine
Clodsire	Machine
Iron Jugulis	Machine
Roaring Moon	Machine
Hydrapple	Machine

Legends Arceus: 22 species

Species	Method	Level
Remoraid	Level up	34
Octillery	Level up	34
Mantine	Level up	37
Mantyke	Level up	37
Psyduck	Level up	43
Golduck	Level up	43
Tentacool	Level up	43
Tentacruel	Level up	43
Buizel	Level up	43
Floatzel	Level up	43
Finneon	Level up	43
Lumineon	Level up	43
Oshawott	Level up	43
Dewott	Level up	43
Goomy	Level up	43
Gyarados	Level up	47
Shellos	Level up	47
Gastrodon	Level up	47
Empoleon	Level up	52
Palkia	Level up	55
Phione	Level up	57
Manaphy	Level up	57

Brilliant Diamond and Shining Pearl: 62 species

Species	Method	Level
Poliwrath	Level up	1
Cloyster	Level up	1
Starmie	Level up	1
Politoed	Level up	1
Ludicolo	Level up	1
Marill	Level up	30
Squirtle	Level up	33
Castform	Level up	35
Psyduck	Level up	36
Remoraid	Level up	36
Mudkip	Level up	39
Golduck	Level up	40
Gyarados	Level up	40
Azumarill	Level up	40
Poliwag	Level up	42
Octillery	Level up	42
Piplup	Level up, Egg	43
Chinchou	Level up	44
Wartortle	Level up	45
Horsea	Level up	45
Wailmer	Level up	45
Relicanth	Level up	45
Buizel	Level up	45
Luvdisc	Level up	46
Poliwhirl	Level up	48
Tentacool	Level up	48
Shellder	Level up	48
Mantine	Level up, Egg	48
Mantyke	Level up, Egg	48
Blastoise	Level up	49
Wailord	Level up	49
Vaporeon	Level up	50
Totodile	Level up, Egg	50
Huntail	Level up	50
Gorebyss	Level up	50
Prinplup	Level up	50
Seadra	Level up	51
Kingdra	Level up	51
Milotic	Level up	52
Lanturn	Level up	54
Lapras	Level up	55
Marshtomp	Level up	55
Staryu	Level up	56
Floatzel	Level up	57
Tentacruel	Level up	58
Empoleon	Level up	59
Omanyte	Level up	60
Croconaw	Level up	60
Pelipper	Level up	62
Swampert	Level up	63
Lombre	Level up	64
Omastar	Level up	70
Suicune	Level up	72
Lugia	Level up	72
Kyogre	Level up	72
Feraligatr	Level up	76
Palkia	Level up	88
Goldeen	Egg
Surskit	Egg
Carvanha	Egg
Barboach	Egg
Bagon	Egg

Sword and Shield: 124 species

Species	Method	Level
Poliwrath	Level up, Machine	1
Cloyster	Level up, Machine	1
Starmie	Level up, Machine	1
Politoed	Level up, Machine	1
Ludicolo	Level up, Machine	1
Marill	Level up, Machine	30
Squirtle	Level up, Machine	33
Psyduck	Level up, Machine	36
Remoraid	Level up, Machine	36
Popplio	Level up, Machine	36
Mudkip	Level up, Machine	39
Golduck	Level up, Machine	40
Gyarados	Level up, Machine	40
Azumarill	Level up, Machine	40
Frillish	Level up, Machine	41
Poliwag	Level up, Machine	42
Octillery	Level up, Machine	42
Tirtouga	Level up, Machine	42
Jellicent	Level up, Machine	43
Chinchou	Level up, Machine	44
Wishiwashi	Level up, Machine	44
Wartortle	Level up, Machine	45
Horsea	Level up, Machine	45
Wailmer	Level up, Machine	45
Relicanth	Level up, Machine	45
Carracosta	Level up, Machine	46
Poliwhirl	Level up, Machine	48
Tentacool	Level up, Machine	48
Shellder	Level up, Machine	48
Mantine	Level up, Machine	48
Mantyke	Level up, Machine	48
Tympole	Level up, Machine	48
Blastoise	Level up, Machine	49
Wailord	Level up, Machine	49
Vaporeon	Level up, Machine	50
Brionne	Level up, Machine	50
Seadra	Level up, Machine	51
Kingdra	Level up, Machine	51
Milotic	Level up, Machine	52
Lanturn	Level up, Machine	54
Lapras	Level up, Machine	55
Marshtomp	Level up, Machine	55
Skrelp	Level up, Machine	55
Staryu	Level up, Machine	56
Cramorant	Level up, Machine	56
Tentacruel	Level up, Machine	58
Primarina	Level up, Machine	58
Dragalge	Level up, Machine	59
Omanyte	Level up, Machine	60
Palpitoad	Level up, Machine	60
Pelipper	Level up, Machine	62
Inteleon	Level up, Machine	62
Swampert	Level up, Machine	63
Keldeo	Level up, Machine	63
Lombre	Level up, Machine	64
Tapu Fini	Level up, Machine	65
Volcanion	Level up, Machine	66
Omastar	Level up, Machine	70
Seismitoad	Level up, Machine	70
Suicune	Level up, Machine	72
Lugia	Level up, Machine	72
Kyogre	Level up, Machine	72
Palkia	Level up, Machine	88
Slowpoke	Machine
Slowbro	Machine
Kingler	Machine
Lickitung	Machine
Rhydon	Machine
Kangaskhan	Machine
Goldeen	Machine
Seaking	Machine
Magikarp	Machine
Kabuto	Machine
Kabutops	Machine
Snorlax	Machine
Dratini	Machine
Dragonair	Machine
Dragonite	Machine
Mew	Machine
Wooper	Machine
Quagsire	Machine
Slowking	Machine
Qwilfish	Machine
Corsola	Machine
Tyranitar	Machine
Exploud	Machine
Aggron	Machine
Carvanha	Machine
Sharpedo	Machine
Barboach	Machine
Whiscash	Machine
Corphish	Machine
Crawdaunt	Machine
Walrein	Machine
Bagon	Machine
Shelgon	Machine
Salamence	Machine
Rayquaza	Machine
Shellos	Machine
Gastrodon	Machine
Munchlax	Machine
Lickilicky	Machine
Rhyperior	Machine
Basculin	Machine
Hydreigon	Machine
Goodra	Machine
Avalugg	Machine
Mareanie	Machine
Toxapex	Machine
Araquanid	Machine
Drampa	Machine
Dhelmise	Machine
Chewtle	Machine
Drednaw	Machine
Barraskewda	Machine
Grapploct	Machine
Cursola	Machine
Pincurchin	Machine
Eiscue	Machine
Arctozolt	Machine
Dracovish	Machine
Arctovish	Machine
Drakloak	Machine
Dragapult	Machine

Let's Go Pikachu and Let's Go Eevee: 18 species

Species	Method	Level
Vaporeon	Level up	31
Poliwag	Level up	36
Squirtle	Level up	41
Horsea	Level up	42
Omanyte	Level up	44
Psyduck	Level up	45
Tentacool	Level up	45
Shellder	Level up	45
Staryu	Level up	45
Gyarados	Level up	46
Poliwhirl	Level up	48
Omastar	Level up	50
Seadra	Level up	51
Wartortle	Level up	53
Golduck	Level up	63
Blastoise	Level up	65
Tentacruel	Level up	65
Lapras	Level up	65

Ultra Sun and Ultra Moon: 82 species

Species	Method	Level
Cloyster	Level up	1
Seadra	Level up	1, 60
Starmie	Level up	1
Omastar	Level up	1, 75
Qwilfish	Level up	1, 57
Kingdra	Level up	1, 60
Pelipper	Level up	1, 50
Alomomola	Level up	1, 65
Castform	Level up	35
Lugia	Level up	37
Poliwag	Level up	38
Squirtle	Level up	40
Psyduck	Level up	40
Mudkip	Level up	41
Gyarados	Level up	42
Remoraid	Level up	42
Tympole	Level up	42
Skrelp	Level up	42
Dragalge	Level up	42
Piplup	Level up, Egg	43
Oshawott	Level up	43
Popplio	Level up	43
Lombre	Level up	44
Milotic	Level up	44
Vaporeon	Level up	45
Chinchou	Level up	45
Buizel	Level up	45
Golduck	Level up	46
Tentacool	Level up	46
Relicanth	Level up	46
Luvdisc	Level up	46
Lapras	Level up	47
Marill	Level up	47
Palpitoad	Level up	47
Poliwhirl	Level up	48
Froakie	Level up	48
Wartortle	Level up	49
Mantine	Level up, Egg	49
Wailmer	Level up	49
Mantyke	Level up, Egg	49
Frillish	Level up	49
Totodile	Level up, Egg	50
Huntail	Level up	50
Gorebyss	Level up	50
Prinplup	Level up	50
Palkia	Level up	50
Tirtouga	Level up	50
Volcanion	Level up	50
Lanturn	Level up	51
Brionne	Level up	51
Tentacruel	Level up	52
Horsea	Level up	52
Octillery	Level up	52
Staryu	Level up	53
Dewott	Level up	53
Seismitoad	Level up	53
Jellicent	Level up	53
Wishiwashi	Level up	54
Omanyte	Level up	55
Azumarill	Level up	55
Primarina	Level up	55
Frogadier	Level up	56
Floatzel	Level up	57
Wailord	Level up	58
Tapu Fini	Level up	58
Empoleon	Level up	59
Blastoise	Level up	60
Croconaw	Level up	60
Shellder	Level up	61
Carracosta	Level up	61
Samurott	Level up	63
Keldeo	Level up	67
Greninja	Level up	68
Suicune	Level up	71
Kyogre	Level up	75
Feraligatr	Level up	76
Goldeen	Egg
Surskit	Egg
Carvanha	Egg
Barboach	Egg
Bagon	Egg
Panpour	Egg

Sun and Moon: 82 species

Species	Method	Level
Cloyster	Level up	1
Seadra	Level up	1, 60
Starmie	Level up	1
Omastar	Level up	1, 75
Qwilfish	Level up	1, 57
Kingdra	Level up	1, 60
Pelipper	Level up	1, 50
Alomomola	Level up	1, 65
Castform	Level up	35
Lugia	Level up	37
Poliwag	Level up	38
Squirtle	Level up	40
Psyduck	Level up	40
Mudkip	Level up	41
Gyarados	Level up	42
Remoraid	Level up	42
Tympole	Level up	42
Skrelp	Level up	42
Dragalge	Level up	42
Piplup	Level up, Egg	43
Oshawott	Level up	43
Popplio	Level up	43
Lombre	Level up	44
Milotic	Level up	44
Vaporeon	Level up	45
Chinchou	Level up	45
Buizel	Level up	45
Golduck	Level up	46
Tentacool	Level up	46
Relicanth	Level up	46
Luvdisc	Level up	46
Lapras	Level up	47
Marill	Level up	47
Palpitoad	Level up	47
Poliwhirl	Level up	48
Froakie	Level up	48
Wartortle	Level up	49
Mantine	Level up, Egg	49
Wailmer	Level up	49
Mantyke	Level up, Egg	49
Frillish	Level up	49
Totodile	Level up, Egg	50
Huntail	Level up	50
Gorebyss	Level up	50
Prinplup	Level up	50
Palkia	Level up	50
Tirtouga	Level up	50
Volcanion	Level up	50
Lanturn	Level up	51
Brionne	Level up	51
Tentacruel	Level up	52
Horsea	Level up	52
Octillery	Level up	52
Staryu	Level up	53
Dewott	Level up	53
Seismitoad	Level up	53
Jellicent	Level up	53
Wishiwashi	Level up	54
Omanyte	Level up	55
Azumarill	Level up	55
Primarina	Level up	55
Frogadier	Level up	56
Floatzel	Level up	57
Wailord	Level up	58
Tapu Fini	Level up	58
Empoleon	Level up	59
Blastoise	Level up	60
Croconaw	Level up	60
Shellder	Level up	61
Carracosta	Level up	61
Samurott	Level up	63
Keldeo	Level up	67
Greninja	Level up	68
Suicune	Level up	71
Kyogre	Level up	75
Feraligatr	Level up	76
Goldeen	Egg
Surskit	Egg
Carvanha	Egg
Barboach	Egg
Bagon	Egg
Panpour	Egg

Changed over time

One change since Generation I

1. Through Gen VPower 120110 from X and Y, Generation VI