Make a taming pen at the bottom of the ocean near a plesiosaur spawning place made of stone 7x6x6 with sealings,foundations,doorframes and a behemoth stone gate.Then use a tough underwater mount to kill the plesiosaurs you don't want, because they spawn in groups of 2 or 3.After that kite the last one into the pen,get out using a doorframe,close the behemoth door and you can hit it easily.Use this strategy to tame plesios and even harder dinos like mosasaur.
These guys are great and super fast and also very strong. Warning when your gonna go tame these guys bring a sarco or an icthy , they also spawn In 2,s or 3,s so be careful because these guys do a crap ton of damage .
They can't fit through the resource caves' entrances. Just lure it into one and pull out your crossbow/harpoon gun for the knock out. Just be aware of any nearby megalodons, as they can fit into the cave easily and ruin your swimming session.