If you have a deck connection for your anchor light it wouldn't be a big deal to tap into that at the plug, with some small diameter wires. You'd have to be careful about keeping out the water, of course.

If you search eBay or Amazon for "underwater LED" you'll come up with lots of options. I'd look for a surface mount design that has a way to route the wires to the side. You could stick it to the deck with 2-sided foam tape, facing up.

This looks like it would work, and it comes in blue or white https://www.amazon.com/Marine-LED-51867-DP-High-Intensity-Underwater-Light/dp/B008YWG3AW/ref=sr_1_1?ie=UTF8&qid=1470753548&sr=8-1&keywords=underwater+led