Ditto on the marine grade wiring, dilectric grease, and good shrink wrap technique.
60 HZ interference should not be a problem, but if your sonar starts having issues with the lights on the same circuit then let me know and I will get you some information on how to clean it up. (Ground loop = bad)
for you techie geeks out there... Wade, i know you know about this...
http://courses.cit.cornell.edu/ee476...w24/index.html