Hi!
Almost the whole map is covered in large amounts of water that lack interesting creatures and features. When you dive into the depths of the ocean you are greeted by well, sharks... Sharks and void. Yes, we have megalodon, mermaids, and krakens, but I think something is missing. I personally don't think that the ocean NEEDS any more DANGEROUS creatures or action-filled confrontations with frightening species of aquatic fauna. Well, maybe, but I think it mainly lacks something friendly. Perhaps just a blue whale roaming the deep blue? Something that will scare you simply from its sheer size, like the Reefback Leviathan in Subnautica. Nothing crazy, just something to fill the empty void of the Sea of Thieves ocean. It would be cool if they could throw themselves in the air and smash rowboats to bits if they were to land on one. Blue Whales might appear in all regions except the devil's roar. Encountering a blue whale would make the ocean a lot more interesting.
My second ocean related suggestion would be the stingray, which is my personal favorite. They live in shallow waters and appear often on beaches. They can hide under the sand and will attack if you step on them. They will use their tail to deal serious damage (like sharks) but only when provoked. This will prevent them from being too annoying as they will not chase you to every corner of the map, and you can also avoid confrontation very easily. My idea is that they will spawn in clusters on beaches, some hiding in the sand. This will make you tread the bottom of the ocean with care. However, they won't spawn in deeper waters. I think this would make a nice addition to the game.
My third suggestion is the Freeze Bomb. Like the Fire Bomb, you can throw it on enemy ships. It will temporarily freeze the anchor, wheel and cannons (whatever it hits), and make the floor slippery. The freezing will not prevent you from using the wheel, only make it turn way slower. Same with cannons and anchor. When a player walks on frozen deck they can expect movement similar to the grog effect.
I hope you find my suggestions helpful!