In order to visit the Ocean Floor , you have to seek out a little creature with a halo shape above it's head, it can be found in the ship's library.
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.
if you take the cabin on the left and try to exit through the broken port-hole, you will find Rusty Key, which can be used in Spirit Town. After getting the key, you will instead find a pair of giant eyes here, perhaps by a cat, which will eat you and jolt you out of your prayer.