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.
He us relativly easy to tame, first one i got was level 15, i used accendant compound bow, with tranq arrows, 10 arrows and it was down, simple, but for a higher level one, place a revival platform in deep ocean, sartound it with walls, stone, metal ect, and revive it, it will take more arrows, lets say 40 if you have a bow as good as mine
guide on how to tame a plesiosaur: I used my 20 k basilo for this, so go up until the plesio stops aggro, if it doesnt aggro after a certain depth. thats perfect, now all you need to to is go a little bit deeper next to the plesio, whistle my 20 k health tame and let it soak. now i used my basilo because of its regen, so it will stay alive for years. anyways, i shot it with my ascendant BOW instead of crossbow because since the basilo is tanking the hits, its distracting the plesiosaur. So I dont really need to move, then i sucked more oxygen from the diplocaulus and kept shooting and finally the plesio swam away, then a few moments later it gets knocked out and boom. you got a plesio ;) (I also used a taming balm, rex kibble because im mobile, and some prime meat and fish meat)
My trusty sarcosuchus made knocking my plesi exceedingly simple, for three reasons. The sarco is fast, it ascends quickly, and you can shoot from its back. Once I separated my plesiosaur from her friend and killed it, the procedure was simple. When the plesi reaches a certain height it begins to flee. I dragged her up then filled her with tranqs until she aggro'd again. Repeat process until unconscious. Simple with sarco.
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.
Taming one is not too difficult. A high level won’t take too many tranq to knock out. When it’s knocked out, feeding it a soothing balm will allow u to tame it quickly, even with regular raw meat. Don’t forget to separate the plesiosaur u want from others in the area. Good luck!
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