Met a four year old on our last trip who had just discovered snorkelling. Couldn't get enough of it, and the fish! He had started at Marina Cay just off the beach: nicely shallow and lots of stripey fish (sergeant majors) he delightedly told me.
Could also try the bubbly pool, provided it is fairly quiet, lots to look for on way there too. Always goats and generally spot a mongoose or two.