No buses in Cozumel. Take a taxi to any of the beaches recommended for about $10 -$15 dollars one way (for 1 to 4 persons). Prices are supposedly standard/controlled, but always clarify before getting in.
I've seen very good things about the following Coz. beaches on this website as well as in the Foders and Frommers travel guide books:
Best best is to visit a book store and either browse for info or purchase book to take along.