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.
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
Trapping- it's a 3 for 1 trap. Trap will work for plesio, moss, Squid. 4 sides of behemoth gates. 4 corner inside trap with pillars higher than gate. 3 Celling on each pillars going center of trap. So only a small gap in center. Now take a small angler or ichy or shark. Lure dinos in and get out from top and close door.
Early game taming trap for big water dino. Now your problem is getting it tamed after that. They take a lot of ressources to tame.
I would suggest plesio and kill other stuff with it like alpha to get black pearls and Lure squid in same trap.
For Ark Mobile: get urself a basilosaurus if u can't unlock the scuba set. Find it and then stay a little bit away from it and it can be done easily with tranquilo arrows and a crossbow. Get some Rex kibble or meat
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.
Since a certain update, you can no longer get the plesiosaur to chase you up until it can't reach. When it can't chase you anymore, it will run away every time. Bring a dunk and let it munch on it against a wall. Especially on Ragnarok, the ocean is very open on there.
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.