Stay on your raft! As long as you don’t put even a single toe in the water, they won’t even know you’re there. Learned this after having a panic attack of a Meg being right underneath my raft. Same thing with any dinos you have on your raft.
I once encountered two megalodons in the shallow water of herbivore island, I just shot at him with tranquillized arrows while thinking that I shot him with normal arrows. He didn’t attack or anything and after a while I noticed that he was unconscious. Tamed him and he didn’t even attack
I don’t think it was meant to be the rex of the ocean more the raptor (Mosa rex, touso giga,) but yes they should be much more dangerous but it would be harder for beginners to try to get a stronger ocean tame.