I had a 260ish direbear get trapped under ice during lag spike and get killed by jelly fish .. the big fker would take down alpha dinos with no prob... killed by jelly fish
Try to tame a Basilosaurus for underwater exploration. They're immune to shock damage from jellyfish and eels, have huge base health and when trained right can fight alpha Mosas. They take pressure damage below a certain depth but the high health pool usually more than compensates for that.
If you want to be killed almost instantly there is 1 cave far worse than any other. It is underwater on the side of the island map. I forgot the exact location due to getting ptsd watching my freshly tamed 150 squid get devoured by a horde of everything killer in 1 small area... It is the only cave attached to the actual island. But just beware...