A transom light (officially referred to as a stern light) as you describe on a powerboat MUST be used in conjunction with a masthead light to be legal. Under the Rules, you are required to have 360 degrees of white light. The transom (stern) light covers 135 of those 360. That leaves 225 degrees for the masthead. No masthead on your boat???? That's why you see the 360 all-around light that is so popular on small boats.
Check this out for more clarity on the subject...
http://www.navcen.uscg.gov/mwv/navrules/Arcs.htm#