We start off in our first underwater level by swimming straight ahead and then moving under the ledge on the left for a full Blue Orb! Once you find the hole to surface, pass the door and get Aquaheart. Then, destroy all the Auromancers before moving onto the glowing ledge above where the heart piece was for a Secret Mission. When you’re finished, move to the right and dive into the hole where you have to reach the surface through another hole directly above. In this room, attack the Sargasso first for they’ll bother you if you try to continue while they’re still there. After they are cleared out, slash the bottom floating sphere and work your way up to the last one to reveal floating platforms above the water level leading to a Blue Device. This will allow you to move on in the previous room. Move through the now open pathway and onto a boss.
This boss is quite a bother to deal with so be prepared and be patient. Usually this boss will be wandering around the water ready to attack you using electric shocks and physical flails. The best advice would be to just keep tossing those cranky bombs and shoot your Bow Gun until it dies. Since it’s in motion, your melee weapons are pretty much useless.
On Must Die mode the battle is much harder by simple virtue of the boss always being invisible. As such the Bowgun (and Nightmare-γ if playing as Trish) is near useless. Your best bet to win is to put Cranky bombs everywhere and hope the boss runs into them. As Trish, DT Round Trip covers a large area and deals significant damage, which makes it incredibly good against the boss.
When it’s finally dead, swim through the doorway above.