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.
If used by a Pokémon holding a Utility Umbrella during harsh sunlight, it does not receive the 50% damage boost, instead being affected by the standard 50% damage reduction for Water-type moves.
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.
Trivia
Hydro Steam is the only non-Fire-type move to benefit from a power boost from harsh sunlight.
This article is part of Project Moves and Abilities, a Bulbapedia project that aims to write comprehensive articles on two related aspects of the Pokémon games.