because of your balcony ? I'm making an assumption that this is your first cruise. For THIS ship my opinion is No... not worth the $$. On the larger vessles you can find quite a bargin to upgrage with a few days on the net, or give your agent time to search... but it seems you're booked, I'd personally stay with what you have and spend the $$ elsewhere... especially since you seem the active type and are possibly looking to just have a relaxing vacay in the room. If you an on the go person, you'll only change, shower and sleep in your room..
Now cozumel is crazy driving in the city, avoid it if it's your first rental in the caribbean, the other roads are more spaced out with vehicles and it's less intense.
If you decde to rent a jeep, I'd search the net like crazy to have a loose itinerary planned with a map to pre determine a few things to see, know in advance how long it will take driving time, add 15 minutes plus for traffic. Also consider where parking is allowed if youre doing a beach for a bit.. you might have 20 minutes added with parking and walking each way... 10 minutes for shopping....
this all does add up and cuts in to your time which is CRUTIAL in a do it your selfer excursion. plus you have to arrive at a rental place, fill out forms, inspect the car... please check that the spare is not flat (long story), then returning can be another 20 plus minutes assuming there is no line. everything works at a very slow pace, 3 coustomers could be an hour wait.
I'm all for a do it your self, but we planned extensively and cautiously prior and still hit scary snags along the way... will do it again... but if it's the first cruise, the extra money spent while you're learning the ropes is money saved from facing really expensive complications.\
to rent a car, I'd catch a taxi on the port to playa del carmen and rent there. If you search the net you can pre book this through avis or what not. Don't be surprised if they don't have any clue you were comming, they should have a rental available and YES, sped to $18 for insurance and NO, do not pay any officer if in trouble to "make it go away". as they might suggest.
also know that if your just doing the beach and shopping, a taxi and good internet searching for a beach is the best way.
I hope this helps
carol