Answers
It depends where the boat drops you off. If you get the ferry to la Savina then there is an abundance of car/moped/push bike hire outlets, take your pick. if you don't have transport then you wont see much from there but the ferry ride over is almost worth it on its own, just to see Ibiza from offshore or the beauty of Formentara as you approach.
Taxi's are an option but not cheap, i did once get a taxi down to Mijorn beach and then asked him to pick me up again 5 hours later at the same spot, well worth it but mucho dinero!
if you get a tour boat across then you may get dropped off at the edge of Illettas beach and have a short wade through the water to shore, you will instantly find yourself on a fantastic beach ( you won't need clothes ), but don't expect any excitement there, its just beaches and a couple of excellent restuarants to while away the hours.
If your serious about seeing the island then get some transport sorted, get a guide book and find your own adventures my friend, you will get much more from the island that way.
Hope this helps.