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.
There’s one underwater cave that higher then normal animals. The devs made it so you
Can’t tame any on the animals in this cave so no one would farm it like crazy. Also if a animal is over level 150 it is untamable.
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.
If trying to tame one make like a wyvern trap on the bottom on the ocean and hive someone else shut it. I use a sarco to lead it in the trap. On the center you can make the trap right next to the air bubble to escaped in the Airr away from any attack range and have a keypad to shut it.
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.
If you're on the Center there's multiple spots where plesiosaurs are glitched and can't move. One is at the redwood forest shoreline near a rock and the other is somewhat in the center islands underwater. The second one has one on top of the other so kill one or tame both.
If you’re on the center, go to the east water dome. There’s a small gap of water between the dome and the rest of the ocean where sea creatures can’t reach. The tricky part is when it starts running away after reaching a certain torpor level but this makes it a little easier.