Steel does ×½ to Water

Generations VI to IX, single types. The other way round, Water against Steel, is ×1.

65 Steel and 154 Water species among the 1025 this site covers.

Attacker, Grass and Steel Kartana sprite

Kartana #0798

damage×½Generations VI to IX

Defender, Poison and Water Toxapex sprite

Toxapex #0748

Steel did not exist in Generation I, so Steel against Water has no value there.

Strongest Steel attackers

Kartana leads Steel at Atk 181

Sturdiest Water defenders

Dual types that change the answer