Was on Formentera on Tuesday, called at the ferry office on the port and managed to get a day return on the car ferry (as a passenger) for 20e which included a free drink out and back; so cheaper than Aquabus then!
Although the ferry look a bit old and battered "Posidon" it's very nice inside, bar (what else!!), comfy seats, plenty of room, good toilets and outside decks for sunning yourself. Interesting mix of passengers from beach lovers to travel-weary lorry drivers. Takes an hour but worth it for the views from the panorama deck, as it's higher than Aquabus, Captain Hook or even the regular jets.