Use something fast that you can use weapons on like a icthy or a sarco than make kill off the ones you don’t want to tame than make one chase you up to where they will start running because you’re too high than it might glitch and go to the surface and pelt it with tranqs.(ur welcome)
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.
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
If you want to tame one I recommend using a icthy because I tamed one by using it. If you are try to pump up its stamina and speed. But obviously if your not like your a sarco master like me🐊🐊 you can use that. Use a crossbow and tranq darts or toxiant arrows because it reloads faster and guns don’t work underwater. Trust me I tried it. It didn’t turn out well. After like 20 tranq arrows it will be unconscious then get somewhere near 10 raw prime meat and BOOM!! You have a plesiosaur /\ if this helped