I have an AquaLamp (I think that's what it's called) in my pool and it has a similar box containing a transformer, fuse, and a GFI outlet (inside the box but inaccessible) that I believe is only used to trip the circuit. It is also designed to be plugged into an outlet.

My box was allowing water inside and it slowly rusted most of the components as well. The transformer was still ok but the fuse had broken in half. I replaced the fuse and sealed the box shut with caulking.

I also removed the standard plug from the box and had it hardwired so that I could operate it from a switch inside the house.