Saltwater Aquarium for Grounded 2

Boss/Unique Weapon

The aquarium would have a Mantis Shrimp boss, they can whack you with their "fists" to create a cavitation bubble of hot air that sizzles you then collapses into a shockwave. They can burrow to launch itself at the player,  or create a blinding colorful flash that blinds the player causing reduced speed and vulnerability to attacks.

Near the Mantis Shrimp is a piece of coral that when broken will let you pick up the Hydraulic Spear. A unique two-handed spear, that can’t block but instead will charge up an attack that spontaneously stabs when you attack, it can’t charge normal attacks though. The spear is very slow, and deals 38 damage(less than the mantis shrimp). It has a 0.1% chance for critical hits and the damage multiplier would be 1700%. And it has a trait called Pressurization, where the more you’re hit the greater your critical hit chance is but once you land one then it resets.

Other Creatures

Cone Snail: they shoot a harpoon like tooth that inflict venom and create a poisonous cloud in the water, though they can also negate damage sometimes.
Pufferfish: puffs up when attacked and will inflict venom on anything that attacks it, though they drop Pufferfish Spines that can be used to make weapons inflict venom by making Venom Extract. Or make the Venomous Arrows cheaper for 85 total. 
Stonefish: a fish that dwells at the bottom waiting for something to get close then it sucks in and snaps, they’re capable of swallowing you then spitting you out dealing massive damage. They drop stone leather used for making many things like stone skin potions.

Resources

Majano Anemones: they take over entire areas and when chopped can give random treasures like pearls used for armor or weapons, usually magic related. 
Seaweed: when attacked will drop seaweed for foods like Sushi or Fish Kabobs. And can be turned into chips that allows for absorbing more water just like the clover set.
Coral: when attacked will drop Coral Chunks and can be turned into armor.                                                Pumice: you can get Pumice Shards from these boulders by smashing them, and they can be used to upgrade weapons and armor after upgrading fully with the brittle materials.

Sorry for any improper grammar, or confusions!