U think the tiny stator can provide enough power to drive the amp that'll be required?
But then, what are you wanting to do with the ski after the sound system is installed? Just cruise around the lake? Get a couch for that... It'd be easier to setup.
That'd be funny to see and hear: a standup setup for sound. Every now and again you'd hear the sound cut out during subs.
Here U go:
http://www.fullwavemarine.com/pwcsoundsystem.html
It's only a 50 watt RMS, though.
Putting a sound system on a standup is like putting a sound system on a harley. The harley is the sound system.