In Pokémon Pokopia, Ditto can learn Water Gun from Squirtle to water plants.
Pokopia turns Pokémon moves into practical tools for rebuilding the environment. After befriending Pokémon, Ditto can learn their abilities and use them to interact with the landscape. Water Gun is useful for refreshing dry plants and supporting habitat restoration.
Squirtle also has a role in the plot. Ditto encounters a dehydrated Squirtle and helps it recover, after which the broader task of restoring habitats becomes central to the adventure. This makes Squirtle important both as a story character and as the Pokémon associated with the watering ability.
The answer is not simply any Water-type Pokémon. Several Pokémon can use water-related moves in the wider franchise, but Pokopia specifically links Water Gun with Squirtle in its early gameplay explanation. That named connection makes Squirtle the single canonical answer.