The bathroom on a boat is called the head. The name comes from the 1700s or earlier. The "head" was always located at the bow or head of the ship, where the wave action would wash off the hull when underway.  Thankfully, this isn't the case today, as most marine toilets store the waste so that you can safely pump it out onshore. But the term has stuck, and no matter what sort of boat you are on, the bathroom is referred to as the head.