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.
Outside of not having saddle armor and being harder to tame, this little guy is essentially an upgraded Ichthyosaur. They give you oxygen like a scuba tank (abd available much much earlier) and are the best creatures for harvesting trilobites. Just don't expect much in the way of combat capability. You can't shoot off the back though, which reduces their usability for underwater taming. Perfect for resource cave runs. As they are amphibious you can take them over land to places other aquatics can't reach, although you will want a flyer to carry them because of how slow they are. Even a Pteranodon will work.
They're also skrunkly little salamanders, it's impossible not to love them.
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