Kelp forests and seagrass meadows are endangered but highly important ecosystem; I think they should be represented properly in the game!
At present, kelp spawns randowmy in the ocean, however this is very different to real life, where huge underwater kelp forests exist. Kelp forests would consist of kelp plants growing next to each other to form a dense aggregation, with limited light and visibility. To represent a real trophic cascade, sea urchins would spawn in the kelp forest, and would destroy the kelp as they feed on it. To counter this, sea otters would kill sea urchins to prevent degradation of the forest. The poor visibility would also make it a good place to have loot; I think that the trident should be made exclusive to the kelp forest, meaning the player would have to visit. A higher drowned spawn rate would also be present, adding to the sense of danger.
Similarly, seagrass is already present but does not form a dense aggregation. Seagrass meadows would have higher light than kelp forests, and different mobs spawning. Turtles and fish would be commin, and this would also be a good place to add in seahorses and manatees.
I think these ideas would make the oceans look nicer, and better represent their threatened real world counterparts.