You're gonna need about three fiddy, then this beast will let you go.

This has to be the most terrifying creature on ark. I've tamed one and it still makes me piss my pants when i see it.

If you are being chased by a plesiosaur,then swim up to the surface.like the mosasaur,the plesiosaur does not swim over a certain depth limit and is never seen at the surface(bit weird since they need to breathe,right?)

If you find one, swim up until it stops going up they will refuse to swim to the surface but they still try to get you. This us how you tame them by yourself