It's not wise to snorkel right off-shore. Way too much boat traffic.
You will see some hotel dock areas that are cordoned off with buoys, these piers normally have some kind of habitat for wonderful little sea creatures, and are safe from boats. Victoria House and Ramon's piers definitely have neat snorkel areas under the piers.
Have the boys start looking up sea horses, octopus, shrimps etc in their books or on-line so they can begin getting excited! Other creatures to look for include the little sponge crabs, bristle star fish, sea urchins, cow fish... the list goes on.
Ohh, you have to go to Rico's at Banyan Bay for lunch one day. Go to the gazebo and throw in some kind of food, then sit back and watch the eels vying for the scraps. Really cool!
Have a blast.