I've wanted to rent a boat in Ibiza for the past three summers but I could never squeeze it into my budget. The Sunseeker is a beast of a boat and will accomodate you and nine mates at a rate of approximately 1,400Euros per day.
Boat hire: Baleric Lizard Can Redo, San Jose, Tel: 620 454 162 or 0034 650 368 593
I can hire you a super fast speed boat which will seat 10, Will cost about 1400 euros, or otherwise a giant catamaran for considerably less which will take you to Formentera and back via Salinas Beach. (It actually surfs up onto the beach, for the ultimate pose!)