To start, I would add some Hardmode enemies to the biome, here are some examples:
+Merman:
- Spawning during Full Moon nights and Rain events. It not only charges at the player, but can also shoot water jets, similarly to the Icy Merman.
- Drops:
- Archer Fish (an early-Hardmode bow, shoots jets of water instead of normal arrows)
- Neptune's Shell (serving as a way to acquired earlier in the game)
+ Coral Elemental: (currently doesn't have a sprite), but it looks like a pile of coral and sand.
- Spawning normally near the surface. It attacks by charging, but it can also dig to relocate to a new possition.
- Drops:
- Coral (4-8) (serving as the way to make coral renovable)
+ Red Jellyfish:
- Spawning in the Underground parts of the Ocean biome, and it basically works as stronger version of the Pink Jellyfish. As for it's drops, I would like to point you to this player suggestion:
Buoyant Glowsticks: The ultimate tool for fishing at night, there are some details I would change about it, but all in all, it's a pretty cool idea.