In Costa Maya, take a taxi over to the little village of Mahahul, just a short ride. There is a dive resort there called Maya Palms with a really good snorkel package. Don't miss snorkeling in Mahahul. The water is awesome.
yes, very easy and Roatan has awesome snorkeling I will add. Be prepared for a 3rd World experience in Roatan. I am not saying you won't like it, we love the island, one of our favorites. But people who are not prepared sometimes come back hating the place.
Belize has good snorkeling as well but we do not like the people as well. Costa Maya, we didn't snorkel there, but you can go to the beaches by taxi, relax and get a really cheap message.