The dossier says they're passive to humans and only attack little fish THIS IS NOT TRUE THEY WILL EAT YOU AND THEY TEAM UP WITH ELECTROPHORUS TO KNOCK OUT YOUR MOUNT AND SHARE IN THE FEAST AFTERWARDS!!
These guys lurk near Caves or deep waters, and won't follow up all the way up (unlike Tuso in mobile 0.0)
Get a fast creature that can surface quicker and wait for the Pleis to follow you until it loses interest. Use a crossbow with tranq arrows (or shocking arrows) for yopor. When it flees, keep doing it until unconscious. I use Sarco and Basilo for mine, and kept feeding both prime fish meat and BIO Toxins (for topor).
If you are being chased by a plesiosaur,then swim up to the surface.like the mosasaur,the plesiosaur does not swim over a certain depth limit and is never seen at the surface(bit weird since they need to breathe,right?)
Not once in all my time on the ark have I seen one alone, there are always two, unless you've already killed one, so watch out when taming, kill one and tame the other or tame both, just remember when you find one that another is probably nearby, ready to tear you to shreds.