The Poly Planer marine systems look very good, like the look of the MRD 70 but expensive with all the extras, but the 630-MP3-KIT-A is not bad :-
A water resistant, entry level MP3 player based sound system kit
designed for yacht tenders, RIBs, and other small craft.
ME-50 50 watt waterproof Amplifier with separate panel mount
waterproof volume control
IC3.5-PM Panel mount 3.5mm MP3 connector for IPod or other MP3
MA-4055 5” coaxial waterproof speakers with speaker wires
ssp £112-95 inc vat in UK or $179-95 in USA