Kill Giant Clams first. They only take damage when their mouths are open.
Near the entrance, there is a left you can take that leads to a note. Reading the note will have a clue to which color the first boss will spawn in (there are 3 colors and if you pick wrong one then you get extra trash).
Stay out of the red circles.
Pirate Captain Madison (1st Encounter):
Kill his two adds first.
Pirate Captain Madison (2nd Encounter):
Always kill the adds first. More spawn at ~25%.
Denn the Orcatoothed:
Right-click Unnatural Ripples when they start bubbling to prevent adds from spawning.