yeah the Astrom is located between the bahia and the riu resorts, very close to the beaches. at the melia you will find speed boats for rent with skipper of course to go there, bring your snorkel gear and some bred and bananas for the fishies, it is a great snorkel spot, but don't go there alone without a boat with skipper, on tide changes the current is dangerously tricky, the skippers know it and tell you when and when not.
as of my knowledge i think the watersports center at the Melia resort sells excursions for snorkeling to the wreck, just ask them at your hotel's beach.
have fun
Mike