Trash

• Towards the start there's a dead end with a note in. The note tells you what colour coral formation to press to summon the first boss.

Chopper

• Burn

Captain Madison

• On the second encounter, summons a pack of dogs that remain even when he's defeated

Denn the Orcatoothed

• You can interact with the bubbling grates to prevent add spawning, but in practice just ignore them and burn the boss