I can't really add to the description of Puerto Morelos that Redforever provided. It's just like that.
If you want to go snorkeling or fishing check out Diving Dog Tours.
They do Fishing and snorkelling trips, evening boat cruises as well.
Kathy Loretta is great, she even offered to take me out fishing for free one time. Check out her Fishing Broucure on the Website below, I'm the guy with the Baracuda!!!
http://www.puertomorelosfishing.com/
You might want to look into Xel-Ha. It's a great place for familes and those that like to snorkel.