If you decide to rent a car, you have many options. Check out
Shore Diving Aruba for a bunch of information on different areas.
While we were there a couple weeks ago we snorkeled at Baby Beach and Boca Catalina. Both are easy shore entries with manageable current. (stay inside the cove at Baby Beach unless you
really know what you are doing) This was my 10-yr-old son's first time snorkeling so we weren't too adventurous.
I have pictures from each location in my
Aruba 2008 gallery.
Haven't done any tours so I'm afraid I can't help you there, but would be happy to answer any other specific questions you might have.