We were on the Nov 9th Western Magic with dd6 & dd4. Cozumel was our favorite port! We got off the ship, walked across the street from the pier to Budget, rented a full size car for $89 (did not have a reservation) and were on our way to Chankanaab within 20 minutes.
In regard to your question about walk-in beaches...Chankanaab has a very nice (but small) walk in beach set on a protected cove (no waves). The sand was great and our dd's love running in & out of the water. We also walked thru the beautiful gardens, watched the sea lion show and had our picture taken with the sea lion (best $5 we spent on the cruise) Then watched people swimming with the dolphins.
In hind sight, we probably spent too much time at Chankanaab. After lunch at the park, we drove a few miles down to Mr. Sancho's beach. What a blast. Absolutely beautiful beach. It was a little steep (dropped off quickly) as you walked into the ocean but was still only about 3' deep as far as 15 yards out. Our dd's loved playing in the waves, making sand castles. DW loved the margarita at the swing bar and just relaxing while watching our girls have a blast. We ended up staying until dusk.
After Mr. Sanchos beach, we drove back to town, dropped off the car and spent about 2 hours shopping in countless number of jewlery stores and then walked back to the ship!
The cabs would certainly have been cheaper for us but we did not have anything really planned and having the car was a luxury well worth the extra $$$.
Hope this helps...Have a wonderful cruise!