Just did this last week in Cozumel during our cruise on the Legend (Crash Cruise 09). It was an amazing experience.
They will have to pay to get into the park and can watch from the shore close to the Dolphin experience desk. Park entry is $16. My friends parents did $19 which included a beer and a tshirt.
If you are going to book this do it on your own and book directly thru Dolphin Experience. We got a great deal at $78 for the Swim Adventure. This was a special and they seemed to be running them constantly. A couple that booked when they docked paid $120pp.
Overall Channkanab is a great place whether or not you swim with the dolphins. Those going along to just Watch will NOT be disappointed. (my friends parents LOVED it).
Carnival Destiny 2011 Cozumel
RCL Monarch of the Seas 2011 Bahamas
Carnival Miracle 2010 Curacao/Aruba
Carnival Glory 2010 St. John NB Canada
Carnival Legend 2009 Western Caribbean
Carnival Spirit 2009 Alaska
Carnival Splendor 2008 Exotic Eastern Caribbean
Carnival Sensation 2008 Nassau
Carnival Inspiration 2008 Cozumel
Carnival Glory 2007 Western Caribbean
Elation 2006 Eastern Caribbean