For use on a boat I would go with the UE Boom. it is extremely impressive for the size, has mounting options (takes standard tripod stud mount (so any bolt on tripod style mount will work for your boat), but it also fits in a cup holder, water resistant and it sounds great. Everyone I have show it to has been blown away. It's better than the Jambox IMO, but not as good as the Big Jambox which is several times larger. The Bose soundlink mini is impressive but again not was rugged. The UE Boom has great battery life as well at around 1/2 volume. It gets really loud and can max out without distortion, I hardly ever have it turned up all the way unless I'm far from the speaker, and it is omni-directional.
The other nice feature is the battery life indication (voice tells how much charge), standard micro USB charging jack, and you can control the volume as well as the music from your phone. Hell if you get 2 of them you can run them in stereo, and the volume would be quite loud. It's louder than my old school boombox in a tiny package, and sounds better too.
It was expensive but one of my best purchases in 2012!