Hydro Steam inflicts damage. If used under harsh sunlight, it receives a 50% damage boost, unlike all other Water-type moves which would receive a 50% damage reduction in harsh sunlight instead.
Bold indicates a Pokémon gains STAB from this move. Italics indicates a Pokémon whose evolution or alternate form receives STAB from this move. A dash (−) indicates a Pokémon cannot learn the move by the designated method. An empty cell indicates a Pokémon that is unavailable in that game/generation.