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.

The weakness of plesiosaur is the depth limit. Use it to tame it.this also work for mosasaurs. Scuba tank and flipper recommended

It's like a fish but bigger

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 pump their speed and stam, you have a underwater transportation rocket

You're gonna need about three fiddy, then this beast will let you go.

I manged to get into a river because these dudes can levitate and don't take damage so the poor bobs saw the unspeakable as the loch Ness monster ate them with this long butiful neck of terror.

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. Voilà. 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.

This has to be the most terrifying creature on ark. I've tamed one and it still makes me piss my pants when i see it.