Years ago, we went out with Colette on 6Paq.
http://www.6paqscuba.com/
While it's advertised as a scuba boat, we took our two young kids on full day snorkeling trip - I think the kids were ages 5 and 8 at the time. She was the BEST tour guide we have asked for - so patient with our kids who at times were frightened of being in the big water. It was a great day trip!