Re: Do boats normally have sinks in the floor?
It's a step.
It is there to enable one to step down before entering the cabin so one is at a better height for the door before going in. Most boats make you duck the door and simultaneously fall into the cabin at its lower height. Most boaters know this little maneuver. It would be nicer to step down before you go into a controlled fall on slick surfaces with low doorways.
It is also stacked and lined up nicely with the other steps going up and over. You can come down and go up in a more fluid motion, even all the way over onto the bow.
Oh, and coming out of the cabin, it helps with the transition going from below deck to on deck.
That's it.
Boating: the only realm where one can confuse a step with a sink. Tune in tonight for the next episode of "What is this thing on my boat?", when the iboaters confuse a glove box with a valve cover.
Ain't boating fun?