LED underwater lights. The ones I've seen are blue and white. I have one of regular ones that I installed on my starboard side as deep as I could get it so it lights up the area where we board the boat from our dinghy. It helps getting on and off at night but the real reason I mounted one is that it lights up the prop and rudder. If I run over a long line or fishing net at night (and there are a bunch of them in Mexico) I can see what I doing to cut it away. It's a full keel boat so I haven't had the problem yet but it's a good feeling to know I have the light. I had to put a thru hull size hole for mine but the new LED's are just a really small wire and the unit it totally sealed.