Key West is VERY easy to do on your own - I wouldn't pay
DCL for that tour at all. You get off the ship, walk down the dock, turn left and you are right into 'town'. You just walk around a corner and it takes you right to all the main attractions. Its an easy 5-8 min walk depending on how fast you walk. You can usually find coupons online for the aquarium and stuff.
I would do it on your own.
Last January in Key West we just got off the ship, wandered the shops and ended up taking the trolley tour. It went all over and you could hop on/off whereever you wanted. The whole thing end to end was about 2.5 hours. I wouldn't do it again, but it was fun once. Lots of history type talk and stuff and the kids on the tour were bored out of their minds.
I think we have decided not to even get off the ship in Key West this time and enjoy an empty ship/pools. It was one of those 'been there, done that' kinda places to us.
Cozumel - it was raining the day we went and quite chilly - so we wandered the mall off the pier and then decided to go out to Paradise beach anyway because it was brightening up. We were the ONLY people there and it felt awkward. lol. We had some lunch and headed back to the ship. The water was frigid and way too cold to swim in

I know you can book snuba tours and stuff on your own easily too. It may be a lot cheaper.
This trip we plan to go to Nachi Cocum for the day. Hoping for good weather so we can enjoy it this time!
Grand Caymen was our fave day - the water was really warm. We took a cab up to the public beach for the day. Good (but expensive) food there and we rented chairs. This time we plan on going to Rum Point as it looks like a much nicer area/beach. I am usually a big advocate for doing ports on your own, but may pay DCL to take us to this one, as it is further out and further away than the public beach. I have to check into options for transportation there on our own.