Sonny, I heard back from one of my friends who went to Stone Island; here's what she had to say:
"We walked to the water taxi from the ship - no more then 10 minutes tops. The water taxi is $2.00 round trip. He just runs back and forth all day - we only had to wait going back about 5 minutes.
Once he drops you off - you just walk down the beach to where all the restaurants are....you can stop anywhere....as long as you order something - even just one beer, you can sit there all day. Beach is not too good for bringing a towel and sitting in the sand - best to have a chair and some sun - it is REALLY hot there (when we were there anyway). It's definately worth just doing on your own instead of paying the ship excursion cost."
I'll have more when I hear from the others.