I'd add an underwater Atlantis-style base where those amphibian COV critters dwell... and merge it with Luminara's suggestion above for a pirate cove. Above the waterline is where you enter the zone via one of the smuggler subs parked at the edge of the island, and you can enter the water and swim below the waves to deal with the things below. And Arachnos mystics have placed an enchantment on the surface of the water here so that anyone going below becomes blessed with waterbreathing and ability to use powers that might require an oxygen environment (fire, for example).
A completely submerged environment is possible, and I recall a small pool in one of the Cimerora mission maps where you can go underwater... probably the original devs experiemnting with the idea that they never followed up on because they either couldn't come up with a system to restrict powers or require breathing or just didn't want to get into it. But with a little magic to handwave away the logistics, you can create a new environment where we can open up potential diplomatic relations with denizens of the deep.
Maybe make it a co-op zone also accessible to heroes through a similar sub entry from Khalisti Wharf.