Marshall, as for other spots you might want to consider...Greece and Crete
If you take a boat from Agios Nikolaos (Ag Nikolaos or St. Nick's) to Spinalonga (or more properly, the Phoenician fortress at Frourio Spinalogkas), among other things you'll pass over is a city submerged by one of the many earthquakes in antiquity, possibly by one in 15 or 1600's that took out Thera. I understand the laws have shifted a bit in the past decade or so, and you should be able to freely snorkel in the area, just be sure not to pick up or remove anything from the site - those laws haven't changed, and are very strict.
I snorkeled off of Pseira, I believe, and it juts up out of the deep water Mediterranean, if you like snorkeling alongside a cliff face in water a couple hundred or so feet deep - with visibility likewise. Closest I've ever been to flying outside of an aircraft, and a pretty strange feeling when the bottom disappears on the way back to the boat..heh! This link provides some info on why I so thoroughly enjoyed my three day breaks from shift at Iraklion AS:
http://archive.archaeology.org/1001/etc/minoan_shipwreck.html
If you want to walk through history, and even snorkel through it, this is definitely the place to go. You can access the island via Souda Bay Naval Support Activity (my last overseas tour before retiring...), but PAX/Space-A flights tend to be pretty full and not easy to find.
Many more spots off of mainland Greece, including a spot in Volos on the East coast near a village called Ag. Ioannis (St. John), near Volos. Beautiful snorkeling, had one of the last remaining Med. seal populations in the region, great fishing. I brought home an octopus from there for my aquarium in Athens. Caught and released a gorgeous SeaHorse (released after showing it to the Taverna owner and his family...used a ziplock bag snorkeling to bring back interesting critters).
Hmm...so much for a "quick reply"...sorry for the book!
Above all, enjoy!