We just got back from this run on 2/28 - 3/4. It was a nice cruise. Because we only had a little time in KW, we went out on our own that day instead of doing a cruise excursion. If you do Duval Street, don't miss Bil Krauss in Irish Kevin's Bar. He's a hoot...lots of fun, clean entertainment. He works 10:30am - 2:30pm.
When we came back to KW for a couple more days after the cruise, we did the Butterfly Conservatory, and walked down by the lighthouse and Earnest Hemingway's house. On the street, there are newsboxes with coupon books. You can find $1 off coupons for the conservatory, or $2 ones which are in the small booklets. Cost for the conservatory I believe was $12 without a coupon, less for seniors.
But your original question was on Cozumel...lol...
We didn't do any cruise excursion, but merely got a cab to the Chakkanaub park. It's $10 (per carload) for the cab ride, and $21.50 per person for entry to the park. They close at 4pm. If you decide to go, the snorkeling gear is $12 to rent. Food in the park is "ok." Nice little man-made swimming/wading area to check for fishies. Nice botanical garden & lagoon. Not sure how the snorkeling was, as the weather had been windy and the bottom was stirred up...dunno if those who were out there saw anything.
One big thing I have to advise: If you plan on bringing cash, get it BEFORE you get off the ship, or get it before you cruise. We made the mistake of using the ATM at Senor Frogs in Cozumel near the pier...the fee was $69.60 (pesos), which converted to around $6 USD. No problem, however, that was apparently just the fee that the owner of the ATM gets, or something. When we got back, we had a $27.85 fee for the $200 withdrawal that we made, so the BANK must have actually had another hefty fee. UGH!
Carlos & Charlie's was fun; a round of 3 yard Bud Lights and 1 yard margarita cost us $59 (again, UGH!)...it was a fun place, though.
Viva Mexico was a VERY cool shop for everything Mexico...I could have spent a LOT in there. They take CCs.
Oh, and make SURE you check your watch, as the ship time is 1 hr ahead of Cozumel local time. If you rely on your cell phone, remember that you need to be back by 8:30 (local) time to make the (9:30) boat curfew for departure.