You will not believe what a difference this small project will make. It sounds almost trivial, but it is a huge plus in making a boat comfortable for living aboard:
Boats are dark anyway. And the closets/lockers are even worse. Is that a light blue shirt or a grey one? Without light in the morning when you are getting dressed for the day, it can be really hard to tell. And in some weird way, having the lites in the closets is welcoming.
The lite you see above was purchased at an RV store (a great source of marine supplies that don't come with the 2X marine cost multiplier). They are 12V, and are meant to be screwed to the top edge of the closet opening. They have a button that, when depressed by the closing door, turns off the light. So that just like your refrigerator, when the door is closed, the light goes out. These are incandescent, but feel free to search for LED versions. Although given the length of time that these lights are on during any given day, the power savings from using an LED would be undetectable.
This really is one you just can't live without.