i've done a good amount of snorkeling. the best snorkeling i've probably done is in hawaii in maui. both at molokini and right off the shore at black rock. saw some rays and turtles off of black rock, and the water and coral is so colorful and clear in molokini it was amazing. there are tons of places right off the shore you an snorkel in maui too and they have maps you can get in the dive shops. i went to a bunch of sites from the shore, but the best was black rock which was right near our hotel.
i also snorkeled in john pennekamp this past september when i was in the keys and saw a 6ft shark which was pretty freaking amazing. that place is also one of the best spots i've ever snorkeled. the coral down there and gardens of marine plants was amazing. they also had moon jelly's which are really colorful and awesome.
snorkeled a bunch in aruba also which was great as well. saw a school of squid which was cool and some rays along with the usual colorful fish. aruba has a lot of great snorkeling from shore as well, and you can also go on boat tours. barbados was good too but nothing special.
bahamas was pretty cool and they took us to one site that had a bunch of reef sharks down about 25 feet and they had a bait box down there, and it was cool seeing sharks kind of frenzy around the bait box. they never came up near us though.
i've also snorkeled in xel'ha when i was in cancun, and if you thought xelha was awesome, well to be honest it's probably the least impressive place i've snorkeled. it was awesome there, but you just don't see nearly the variety of fish, and hardly any plants and coral. also the brackish water was kind of weird because you had to swim in paths or you looked like you were in blurry water. i snorkeled when i went to xcaret as well which was pretty lame because it was in cenote's and it was too crowded. the best snorkeling i did in mexico was on a boat tour off of the cost of isla mujeres.
i'm going to turks and caicos in may and plan on doing a bunch of snorkeling there too.
if you want more specifics about the tours i took i'd be happy to give you more info. i also have tons and tons of pics because i have an underwater camera. snorkeling is probably my favorite thing to do like ever. i've done it for hours on end before without taking breaks.
In Aruba, there's the sunken wreck of the Antilla you can snorkel and scuba dive to. It was a German cargo ship scuttled in 1940 to avoid capture. It's in pretty shallow water. Done it about 10 years ago. It's formed an artificial reef so it's neat to visit.
it's actually not shallow there at all. it's like 50 or 60 feet deep, but it's cool to see the ship. it's also not really a beginner snorkel place because the water is very choppy there usually, unless you get a really good day. that was the one site that they made everyone wear life vests both times i've snorkeled there, but at least they were the kind you have to inflate so you could basically snorkel without feeling like you were wearing one. but usually the ones that take you to the antilla also go to 1 or 2 other spots as well.
EDIT:
here is a thread i made after i got back from the keys with some pics/vids. the keys really did have some great snorkeling and it's right here in the continental us.
http://forums.anandtech.com/showthread.php?t=2344566