I remember there were loads of companies with water activities at the San Antonio port - diving companies, boat excursions to all over the island and the sunset cruises, parasailing...you name it, you can find it there. They'll be fighting for your patronage so you may be able to score discounts. It would be a great place to start. I tried booking excursions before my trip and found that they were less expensive if you just went and booked them while there. The German boat trip to Atlantis was a lot of fun and I was able to book it pretty cheap when I was in Ibiza - and they were very clean and decent Germans, thank you very much! LOL