Terry, not a Marine but in the USCG. Same maritime terminology/jargon is used.
Two possible explanations for "Head":
1. From Wikipedia: Head: Bathroom (The term comes from the days of sail, because wind would blow from the rear of the ship foreward the bathroom would be located at the front "Head" of the ship to carry the foul smell of excrement away from the crew).
2. From another site: The "head" aboard a Navy ship is the bathroom. The term comes from the days of sailing ships when the place for the crew to relieve themselves was all the way forward on either side of the bowsprit, the integral part of the hull to which the figurehead was fastened.
If you think about how a sailing vessel is propelled, i.e. wind pushing the vessel from behind, having the head at the bow makes sense.