If attacked with a Boat Cannon or the Sea Legs while still sane, Swimming Horrors can take damage and die. this makes it possible to collect Nightmare Fuel without needing to go insane. This might also extend to Coconades but haven't tested.
Steps to Reproduce
Attack a Swimming Horror with a Boat Cannon or Sea Legs while being between 15 and 50% Sanity (When they are neutral)
Attack a Swimming Horror with a Boat Cannon or Sea Legs while being between 15 and 50% Sanity (When they are neutral)