I was traveling alone and took a taxi to Chaankanab. I beleive it was $10. Just remember you have to cross the walkway over the street, through the shopping mall and down to the taxi stand on the other side.
There were vendors trying to sell things, including tours, all the way to the taxi stand. The cabs are regulated and the drivers were helpful. Just remember what cruise dock you came from as there are more than one.
Chaankanab was quite nice. They had palm covered umbrellas shading beach chairs. Drinks, snorkeling gear, underwater cameras and such were readily available for a fee. The snorkeling is very accessible and beautiful, both fish and coral.
I did get stung by a jellyfish but after a short discussion in my broken Spanish and his broken English clarifying that I was not likely to die, or even be terribly inconvenienced, I allowed the lifeguard to spray vinegar on the stings and I was back in the water 20 minutes later.
There were iguanas everywhere and the flowers and vegetation were lovely. Also many reproduced artifacts and ruins with explanations about the Mayan culture. I stayed most of the day and never even got to the dolphin area. I'd think the dolpins would be a must do with children though.