Update Aquatic already has revamped the game in so many ways. But it still feels barren and dull. The two things imo that the oceans lack is biomes and more aquatic mobs.
The ocean has a lot of biomes, but seriously, the only thing that is only recognizable is the warm oceans biome with corals in it, the rest were uninteresting. Since the oceans has a lot of trench-like openings, why not add a new deep ocean biome with bioluminescent mobs and new mechanics, like adding water pressure specifically in deep ocean biomes is now a thing.
Also, more aquatic mobs. Don't get me wrong, tropical fish has already like 1,000+ variants, but honestly, warm ocean biomes are uncommon to find. I'm thinking a fish that sits between cod and salmon, maybe tuna fish or swordfish. Also maybe add whales (and sharks) because its fun to encounter something big in the ocean, like whales occasionally expose themselves in the air to blow water (since dolphins [at least for me] doesn't really jump out of the water). And more aquatic mobs that only lives in the seafloor, which could be starfish, maybe a small biome where a large portion of jellyfish is concentrated, crabs, or clams.
My main base in my forever world is situated near a massive ocean, and I always travel in the ocean so I couldn't help but notice how dull the ocean is, my point is, just makes the oceans more livelier is all.
The Update Aquatic is already great though, don't get me wrong, but they could push more.