There is a glitch at the redwood on the center, where plesiosaurs will spawn really near the coast and whole groups of 2 or 3 are glitched in the water, meaning you can knock out multiple plesiosaurs at the time.
The trick is to argo one, have him chase you into an underwater cave where he will not be able to fit through the door but his long neck and head does. when you do this he will not de-agro, you can then get a clear shot from the cave with a bow and tranq arrows
After tamed, you can grapple onto them while a tribmate drives it and easily tame more large water mounts. If you are about to touch a structure or terrain, crouch to take off the hook or you will die instantly and your body will fly across the map
The trick to ko these guys is to wait for them to agro you, once they're close enough, swim up and backwards until you've reached their swim limit and shoot them as many times as possible. When they lose interest, just do the same trick and eventually you'll knock it out
There is an invisible barrier at a certain depth that the plesis will not cross, you can drag them up to this and shoot them till they deagro, when they do reagro and repeat until their knocked out. Also, you can drag them to the entrance of an underwater cave, and shoot the plesi, as long as you dont venture too far into the cave, it will not deagro.
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?)
A beast of burden to the ocean as the bronto is to land, the plesiosaur can open many opportunities to the ocean. A good carry weight and large platform make it able to carry loads across the sea, or mobilize your naval superiority against your foes.
If you're looking to tame one of these, it'll be interesting. Have a fast and strong megalodon to ride on so you can get around fast (or a manta/itchy). They can't go up much because of a sea barrier keep that in mind. Use that to your advantage! Tranq arrow it (bring a spare bow if using normal gear) and make sure you watch your oxygen levels.