I have a bookshelf unit velcro'd to the wall above my settee. It's got a radio, CD, and iPod port. At anchor it's more than enough. At sea it's enough for some background music.I'll second no holes in the boat... bookshelf speakers are better, and magnetically shielded speakers better still (see comment about compass and magnets).
I've found marine speakers are usually power hogs... 50w peak is probably barely enough to hear any distance (over rushing wind/water noise while doing 6 knots)... I had to put an additional 100w amp on mine, but then my bookshelf speakers are below.
I just spent three days on my boat with my wife and kids. Sailed Saturday and Sunday. I'm usually onboard mid-week to avoid the crowds. I think you may have me confused with another boat...You wouldn't know if there were holes in you boat YOUR NEVER THERE...