Before I put my boat away for the winter, I was giving everything one last look-over.  When I went to check my lights, they wouldn't work.  Everything else was working just fine.  I did find a mouse had been in one of the compartments, but did not appear o be anywhere near the exposed wiring.  Is there any way to diagnose my problem easily?  If the wires are chewed up somewhere under the deck, is there an inexpensive and easy fix?

If nothing is lighting up, it sounds like an open main ground connection. How are you testing?

I would use an ohm meter. you can use a long peice of scrap wire as a jumper to ohm between switch and light.

Ck fuse or circuit breaker.

Kelley

Since both the bow and stern light quit simultaneously makes me think it is an issue with a fuse or connection at the ground or connection at the pannel.

First I'd check the fuse.

Most likely if it was just a short in wire one of them would light (unless it is the main supply to the panel, but then nothing else would work).   

If you have a short (i.e. bare wire) in the positive it would also probably keep blowing fuses(or tripping a breaker).   So if you replace the fuse and it keeps popping you got bare wire contact somewhere most likely.

Next I'd check/clean the ground at the battery and/or pannel.

If you know how to use a multi-meter or ohm meter they are great for diagnosing the problem.  I can't tell you how many times that $10 investment has saved me a lot of time and frustration.