I made some lights a couple of years ago. They were still working but they were pointing down in my engine bracket which made them hard to see and they used 50W halogen bulbs which used a lot of battery power.
This time I went with some high power LEDs from
www.superbrightleds.com. The first pic shows them as they came. This time I made an acrylic case for them and mounted them on the bracket between the engines pointing back about 3" below the surface.
This is how they came, they are about 1.5" square.
Here is how I put all three as one. To jump the power I soldered wires to the socket plug and coated it with liquid electrical tape. You can't cut the bulb socket off, thats where the electronics are. I started with 4 lights.
Now building the case. I have been in the aquarium buisness for many years and have built a lot of aquariums out of glass and acrylic. This case is basicaly a tiny aquarium sealed.
The finished product, the coke can is to show its size. No way to get in to it now without cutting it open. I didn't want to worry about leaking seals.
Here it is mounted on the boat.
The pics don't do it justice, these tiny lights are amazingly bright, I can't even look at them directly without my eyes hurting. It should look sweet in the water.
And it uses hardly any power, each little square is 190 miliamps which is a total of 570 miliamps. There are 1000 miliamps in one amp which makes this entire light just over a half of an amp. I think the 50W halogen were 4 amps each.
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My homemade underwater lights. 
