In 25+ trips to SP we've only had the problem one time. If you cancel because you just don't feel like it that day, or your group decides otherwise, that's not fair to the operators.
Last November we had a private cat sail scheduled for the afternoon, but at 12:00 noon the sky was dark, really dark, and lightning was striking at the reef. Three of the four of us are USCG certified captains for private sailing vessels, and the decision was made to cancel. If I have a CHOICE, it would not to be on a vessel with a aluminum 30' mast in a lightning storm. A fairly sane option we decided. The operator, for a time felt otherwise, but in the end conceded that common sense (and the potential adverse publicity) was not worth the $200.
This happens very rarely, in fact most of the weather related cancellations in our experience are instigated for good cause by the local operators.
Bottom line...listen to Barb.... no matter how tempting, don't prepay a bunch of diving or excursions.