They do have jet skis at Paradise Beach, and there is a taxi stand there. I guess it would depend on how much time you're going to have in Cozumel. The swim with the dolphins takes about an hour. If you want to watch the video and purchase your pictures, that will take about another hour. It will take about 20 minutes to get to Chankanaab Park, and you might decide to stay there and snorkel or just check out the park. Normally, you're in Cozumel for only 8 hours. It'll take a while to clear the government paper work, feeding of the boarding officers, etc., so that will cut into your acual time in port. And you have to figure that you'll need to be back to the ship at least 30 minutes before they sail. I don't like renting jet skis, but we have done it on the ship's private islands, and a couple other islands like St. Thomas and Isla Margarita. I just don't feel safe about it, however, I would think both Cozumel and Grand Cayman would have as good of maintenance as any place in the Caribbean. They are big tourist areas, so I would feel pretty confident renting there. Just don't stress yourself trying to do everything at once. That's one of the best reasons to book another cruise, do what you couldn't the first time (as if I need an excuse)
Hope this helps.