We did jet skis in Costa Maya. There are booths set up in the little shopping village where you can book them. They get you a taxi that takes you to a little private beach club (just a beach area with a bar/restaurant) where you can jet ski. I don't know if there are any you can book through the cruise or not. This was a last minute decision so we didn't check it out. We also did some snorkeling there and just enjoyed the beach area then had the taxi take us back.
Carnival Valor - 10/2008
Carnival Sensation 10/2006- now Fantasy
Carnival Miracle 10/2005