Level movespeed to make it faster on land and in water. I tested with 2 of them same level. One max o2 one movespeed. The one with maxed oxigen (level 521, 60k+ o2) got no speedincrease on swimming. The other with 171% speed was like 3x faster in water.
These guys are the kings of trilobite hunting. Not only do they deal additional damage to them, they're great at collecting chitin, oil, and pearls. Other perks: they don't need a saddle to be ridden while in the water, they provide you with oxygen underwater, and are faster than just about anything while swimming. Super underrated mounts imo.
Amazing for starting out if you wanna go in the ocean and you don't have scuba gear unlocked, they are extremely fast and can outrun pretty much anything in the water like the dolphin, I still use mine even now