Flukemarm

The Flukemarm is an optional boss in the Royal Waterways and the source of all the Flukes infesting the area. It guards the Flukenest charm.

Attacks
The boss floats in the center of the massive room, completely stationary, and continuously spawns quick and aggressive Flukefey from its orifices, two at a time. You can know which orifice is going to spit out an enemy by looking at the bile the Flukemarm is spitting out.

Tactics
This boss doesn't attack, but spawns enemies that are really fast and frantic and can easily overwhelm you. There are two possible tactics for the battle.

The "offensive" tactic requires you to have a Nail upgraded enough to kill Flukefey in one hit, or Fragile Strength. Focus on killing the Flukefey as quickly as possible, and hit the Flukemarm whenever you can. This can be hard as the Flukefey are really difficult to hit.

The "defensive" tactic involves using Quick Focus and Stalwart Shell to buy yourself the time to heal, Soul Catcher and Grubsong to generate enough Soul to heal, and Thorns of Agony to help kill the Flukefey and occasionally damage the Flukemarm itself if you're close enough to it. This tactic works even if you can't kill the Flukefey in one hit. Focus on the Flukefey to avoid becoming overwhelmed, heal as often as possible by using the window provided by Stalwart Shell and the Quick Focus, and only attack the Flukemarm when there's an opening.

The most important thing to avoid in this fight is falling into the water. Since you cannot attack while swimming, you will be left completely defenseless until you can get out of the water, and the Flukefey will likely get hits on you.