Dive lets Ditto explore underwater areas in Pokémon Pokopia. The ability was added in a later update and opens access to otherwise unreachable regions of the map.
The move is especially important for Bubbly Basin, the first part of the game’s three-part Expansion Pass. That content introduces an underwater village, aquatic Pokémon, and water-themed furniture, making underwater travel a central part of the new area.
Dive should not be confused with Surf, Waterfall, or Whirlpool, which are also water-related Pokémon moves. In Pokopia, the relevant feature is specifically the ability to go beneath the surface and reach underwater locations introduced by the update.