I manged to get into a river because these dudes can levitate and don't take damage so the poor bobs saw the unspeakable as the loch Ness monster ate them with this long butiful neck of terror.

Put a grill on platform. Farm other plesiosaur for prime meat. Cook underwater

If you pump their speed and stam, you have a underwater transportation rocket

Beefy, powerful, fast, good lifter, supports platforms, can be tamed on foot with much less efort and risk than Mosas (still not easy). For sure this thing is the most versatile mount after Mosas, Quetzals, Wyverns and Mantis (oh boi, you NEED a Mantis).