[SHNOOL]

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.

 

[SHNOOL]

It's a hunter 23 guys, it's not really an offshore boat here.
I guarantee, his boat is quieter than THESE guys on his inland lake:

 

[SHNOOL]

I did the bluetooth route for a while as well, it was a decent alternative.... use a good one though (like shown above), I bought cheap and got what I paid for.