pokemon like dragonite, lugia, and rayquaza all share this ability to fly through the air and dive deep under water. lugia and dragonite are even nicknamed "the guardians of the sea," with lugia itself being found deep under water.
now, the idea is land or air-based pokemon that can learn surf yet are not water mounts being able to swim. two new mount types: amphibians (pokemon like blastoise or nidoking) who are able to swim and walk on land at normal speeds.
and avian amphibians (such as lugia and dragonite) who can swim the deep depths of the ocean and soar above the clouds.
all of these being unable to buck you off in water.