kiddisney,
We've been on two previous cruises in September, and it was still quite warm in September. You just have to figure that the Caribbean is going to be warm whenever you go.
You can go to
Weather Undergroud (click here) and see a history of the weather in Cozumel and Key West. I picked a random date in July, and the high temp in Cozumel was 91 and the high temp in Key West was 89 (with the record high being 93). No 100's to worry about.
Also, the ship is more than adequately climate controlled, and even when you are shopping on the islands, the shops are almost always air conditioned.
And of course, the water at the beaches is the perfect temperature to cool you off during a day of snorkling, swimming, or just lounging!
So, I wouldn't worry about changing your cruise just because of the temp. The adult pool has a great sunshade around it, or you can sunbathe on deck 10 and get the great breeze from the moving boat (sometimes quite a stiff breeze, depending if you are infront of the wind diffusers).
It's going to be a great cruise. If you need any other advise, feel free to ask. The great folks here on the Disboards know all the answers! (Plus, we might have a few tips, as well!)
See ya on Deck!