Snorkelling in Aruba is best done on an excursion. They will normally take you to a reef off Palm beach and to the two offshore wrecks. Alternatively, there is good snorkelling if you go to DePalm Island or outside the breakwater at Baby Beach.
St Thomas-Best snorkelling is said by most to be over in St John. Many go to trunk Bay. If you want to stay on the island try Coki Beach or Saphire Beach. Maegens Bay is a beautiful beach but it is crowded and snorkelling is not great. Good snorkelling can be had at Buck Island (an excursion offered off most ships). This is NOT the famous Buck island. Most folks say the best snorkelling in StMartin's is at Pinnel Island. Usually available as an excursion off the ship. Sorry, can'y help with Curacao.
Presume by the islands you asked about you are doing Adventurer of the Seas .
our daughter lived on St johns for a short time & recommends anyone who is down there to definitely visit St Johns as it is much more serene than St Thomas - Trunk Bay is very busy but beautiful -take a ferry to St johns from St Thomas - my sister just got back & from Trunk bay - she took a taxi to Red Hook & then ferry over & spent a few hours at the beach snorkeling at Trunk Bay - She said it was beautiful!! my husband & I are going on Constellation 4/12 & can't wait!!! loooking forward to R & R
Trunk Bay from a boat is an outstanding snorkel trip
Aruba has some good shipwrecks but the water was freezing cold in June
like the lifeguard on the catamaran we were on had a wetsuit on
St Maarten Pinel Island was disappointing to me