Underwater Ship

A sunkern wreck that can be accessed by taking the lift that's in the shadowed house in Sunset Cliff.

In order to visit the Ocean Floor, you have to seek out a little creature with a halo shape above it's head.

Interact with it and it'll disappear, if you then head down further into the ship, you should eventually get to a wide area where the path splits in two.

Go to the cabin on the right, you should find the little creature from before, interact with it and it'll take you to the Ocean Floor.