I too wanted to do this last year when i had a ski boat, bought the speakers (JVC waterproof model) and a waterproof front for a standard car radio which is really just a sealed and hinged plastic lid to fit on the 'dash'. Never got round to fitting it though...
Now that i have been 'converted' to a real boat i would still like to have some background noise at times and have considered fitting the outfit - but so far have not really found anywhere to put the speakers.
My other concern has been over what 'headunit' to use - will a CD player really be of any use at-all? I think not! Tapes are dead, and i think minidisk would suffer the same fate as CD, so what i am really waiting for is a solid-state in-car MP3 player.
I must say a second whip aerial will look good on the A-frame though :-)
The waterproof cover for car radios is made by a company called 'access technologies' and the model is an MRCV-100. I got it at the London Boat Show '01, for about £40 i think. From what i remember they had an alternative full enclosure available, but i don't know who i bought it from i am affraid.
Let us know further detials if you go-ahead...and where you intend to fit the speakers (top of my fuel tank is under seat and front of console has an access door).