An option I am investigating for my cockpit is the use of tactile transducers.
Here are a few links to a few pages in the Parts Express Catalog:
The item here appears to use a rare-earth neodymium motor, and should therefore be inherently shielded magnetically:
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=300-377
The item here is designed for extreme environments and is sealed for a IP67 for truly waterproof performance:
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=300-376
For my C-22, I am investigating a few locations, specifically the inner transom skin, the vertical panels under the cockpit seats, and even the inner walls of the coaming storage pockets. Large thin flat areas of fiberglass that are not cored will give the best performance.
Using a tactile transducer on a thin non-structural panel could provide reasonable full range sound without cutting any holes. Attach the device and let it vibrate the fiberglass panels, as if they were loudspeaker cones. I have heard similar devices in other applications, and have been rather surprized. You won't get the highest or highs, or the lowest of lows; and there will be some amount of coloration of the sound. The coloration is dependant on the panel you end up attaching the transducer to for sure. It is not hi-fi, but still, it can be surprisingly good given the right application...