There are literally thousands of option for lighting up the interior, so it might help if you give a few more specifics. Are you wanting just a bit of courtesy lighting, speaker rings, completely lit/blinged out, etc.? What kinda budget are you working with? Single color or RGB?
Old threads on underwater lights:
http://www.wakeworld.com/forum/showthread.php?t=801455
http://www.wakeworld.com/forum/showthread.php?t=702750
http://www.wakeworld.com/MB/Discus/m...tml?1239753762
http://www.wakeworld.com/MB/Discus/m...tml?1247156041
As for the underwater lights, there's not really a "cheap" option out there. Even the "more affordable" stuff seems to be a couple hundred bucks, so be prepared to drop some coin on the lights, and then a good bit more on getting them installed and sealed properly since almost all of them require you to drill a hole of some sort into the back of your boat. This is the part you don't want to cheap out on unless you want to see what those lights look like shining up from the bottom of your lake.