- Start with a baryonyx, as a guy here mentioned, they have a stun attack that makes swarms of manta, dunkafish and dolphins a lot easier to deal with, they are a ground tame for the most part so no scuba shenanigans required.
- Baryonyx stun attack can also take out jellyfish if you are careful, without you getting stunned yourself. (There are easier ways to get rid of jellyfish, but its an option)
- Use the water tame ceiling with taming plesi's and mosa's they only rise so high in the water and will go back down, you can safely shoot them from above a certain water height.
- Always take a second scuba tank, flippers and mask, they break easily.
- I play PvE unofficial so I have a lot of points in oxygen, I'm faster in flippers than almost everything in the ocean, being faster is the best defense as everything big in the ocean turns like a battleship, you can keep away from the bitey end.
- Avoid the sea scorpions off a mount, they can knock you out in one sting.
- Scuba tanks don't work when you are asleep, you could potentially drown if you are out for too long.
- Basilosaurus are the "meta" right now, as a passive tame they are relatively easy to get with raw prime fish meat (mutton if you have it works amazing) They have so much health and are immune to eel and jellyfish stuns, they regen a lot of health very fast near the ocean surface, I'd say after a baryonyx to get you in the water, they would be the next tame in progression.
ARK Survival Evolved: Underwater Taming