If there are enough ambient passive mobs in the ocean, next to places like ocean monuments, the Dark Waters mobs are too distracted by hunting them to aggro me right away. In fact, they don't even need to be water mobs. I spawned a manaraw for testing in a natural pool in a desert biome, and it aggroed onto a rabbit, climbed out of the pool, and suffocated to death. It didn't even hurt me. The problem gets worse if more modded passive entities are spawning in the ocean. Due to the monsters killing a lot of sea creatures, there are a lot of (admittedly useful) items floating on the surface that I can just pick up, but at the moment the monsters are not adding enough spookiness to my ocean, they are simply slaughtering squid.