Here are 2 locations I would recommend.. relatively mild current when I was there, and plenty to see. These are based on 1 trip in April for 11 days with my family, including a 15, 13, and 7 year old.
Dzul Ha - Nice snorkeling.. this is a popular stop with snorkel tours.. no need to pay for the tours, however, unless you wish for lessons
Chankanaab - 10 buck entrance fee per person, and can be crowded at times, but our family enjoyed it.. tremendous number of fish, some very interesting artifacts underwater to look at.. can snorkel over divers (shallow water, app 20' deep about 50 yards from shore). We sat through a Park Royal timeshare tour for free tickets.. we got free breakfast, and tickets for 5, for an hour of our time.. worth it.
As for wrecks... I am not familiar with any first hand, though I have heard there is a plane (not very well preserved at this point) off of the beach across from the Park Royal hotel (not sure of beach name)...
We stayed at Cantamar condo's, north of the city, and frankly, the snorkeling there was terrific.. fewer crowds, much to see (2 eagle rays on our first day, none after). I am not where the best public access to the water would be around Cantamar, but the snorkeling, and lack of crowds was great. The water is about 12 to 15 ft deep, and the wall along the shore contained a tremendous amount of life.
Have fun.. we had a blast diving, snorkeling, and relaxing.. so much so I am taking my now 16 year old son for 10 more days of diving in August..
Paul
