You can check out the beach guides located at
http://www.caribbeanportreviews.com/Cozumel.htm
for Cozumel. Simply scroll down to the special reports on the various areas beaches are located and then check out the ones you might be interested in. There is everything from a cruise party beach to solitude perfection, all within a taxi ride away.
Also check out
http://www.caribbeanportreviews.com/CostaMaya.htm
for Costa Maya and read Nancy Norris' review of Majahual Beach. Hope that helps.
Have a great cruise!
Tom