What it could be : Ocean Tunnels could be more like a Bee Hive in the jungle, with it having a block that the player destroys in order to summon a boss or event. There could also be something like the bees that spawn in a way similar to breaking those blocks, or better yet breaking the eggs in the underground dessert biome. We need more biomes that scream "THIS IS WHAT I AM" like the Bee Hive does.
How this would be done using bosses : The way this could happen is Surrounding it with corals and giving it unique background walls such as shell ridden sand or something similar. And in terms for the boss it could be similar to the Queen Bee. What I mean by this is it could be a colossal squid that uses ink based or coral based projectiles. It could align to shoot a laser in a large circle as the boss rotates, effectively being like MoonLord's eye laser attack. It could have suckers that brab on to the player or it could be like the Flying PirateShip in the way that you have to destroy the tentacles not the boss itself. Another option could be to have a jellyfish boss that uses electrical based attacks. Such could be an attack that is based around the jellyfish boss that stuns and hurts the player if they are within a certain range of the jellyfish boss, or arcs of current that, upon reaching the player, explode into balls of electricity. Since there should be a variety of attacks there would also be slow moving ball of electricity or electricity mines, and a fast attack like the before mentioned attack for the colossal squid boss.
How this would be done using events : The player could be granted an event only buff that grants them the ability to breathe underwater, or when an enemy dies it could release an air bubble allowing the player to have their breathe regained. Enemies could include the earlier mentioned bosses, drowned versions of other enemies, a larger crab variation, or even water elementals or other water based enemies. It can also give the player water based loot, weather it would be an item that gives the player buffs, or a usable weapon or accessory. An item that when equipped gives the player stat bonuses when in water, like the SeaShell. Or a throw weapon like water balloons that would do splash damage. Or, and this is a stretch, a new summoner weapon that summons an eel that gets longer the more you summon it, similar to the StarDust Dragon Staff, except maybe late Pre-HardMode or early HardMode, along with a new light pet like a jellyfish that follows you around, and when in water grants increased light! And maybe even go as far as make an item that grants water breathing.
What else this could offer : #1 : It could be yet another dungeon like biome that presents a long winding path all the way down to hell or near it. #2 : it could be an abandoned underwater town with some loot or even already viable NPC housing. Along with, and since the final large update came out I doubt this, but another water based NPC. Maybe something like the "Mr.Fish" in reference to SCP Dr.Wondertainment that could sell water related items. This NPC could sell a variety of glowsticks or different colored water torches, and maybe even a few new water accessories.
These are my thoughts on this amazing opportunity biome. Please be sure to tell me if you liked my ideas and feel free to add on to or otherwise change parts of my ideas to suit the gameplay more!