as a snorkeler, save some $$ and skip the blue hole. in 2 trips, i have not bothered.
shark ray, hol chan, mexico rocks & the aquarium are all super snorkel destinations. i suggest you bring your own mask & snorkel.
i do not think there are atm's. first trip i did travelers checks, second trip just cash & plastic. the better restaurants and most gift shops take plastic. all are happy to take your US dollars or traveller's checks, and most hotels will change them. there is a bank in town.
always bring bug spray, ya never know. hiking boots are unnecessary. i don't know of the plane thing. i paid $84 US last year.
bring lotsa ziploc bags. they really come in handy.