We decided to go on the 3 hour boat trip to the bottom of Ibiza and had a fantastic time (despite being a little cold) They show you all the caves, coastline, bays/beaches and island and lets you know some of the history and some fun facts.
As you board, they take your photograph (which I wasn't expecting) which they ask you if you wish to buy nearer the end of the cruise. You also get a free glass of champagne, however they also see other snacks and drinks on board. There is a great opportunity for a swim in the sea. On the day we went we also happened to see Dolphins playing about in the sea. The pilot decided to circle a few times so everybody got a chance to see them, which was fantastic.
Your Tour Operator (Thomas Cook, Thomson, etc) may offer an excursion similar, but don't book with them. You can save yourself a bit of money if you book direct with the Captain Nemo team. The price is cheaper and you get more for your money.
An excellent day trip, well worth the money.