If there is anything I learned is that it is better to get off at the second stop (after Ses Illetas, the boat continues on to Port de la Savina). There are no rocks there...
Then rent yourself a scooter, explore, grab a bite to eat, check out the beaches, etc. You can get back on the boat at the port too, about 15 minutes before it picks everyone up at Illetas. If you're skiddish about scooters, get a bike or take a taxi or bus to the beaches.
A one day trip is just enough to figure all this out but not enough to do it. I'm definitely glad I made the effort though, finally, on my 4th Ibiza trip.