If you're interested in somewhere that isn't tropical, then the Medes islands, Spain, is one of the best. We went to the Med last week and went from l'Estartit across by boat. The group hired my family wetsuits, fins, masks and snorkels and after a boat trip of only 10 mins we were in sea grass meadows with cuttlefish, sea cucumbers and octopuses, and then along the rocky, Mediterranean reefs with sea urchins, sea anemones, and too many fish to count, hundreds of species. There are also huge groupers and seahorses to see. The water was amazingly clear and the fish variety was unbelievable
This was an amazingly practical trip for me as I live in the UK, and I'm going back there next year to dive, but if you're living in the USA then maybe it's not the best trip for you.
If you did just want a diving trip then I do highly recommend the UK. I have dived in the North sea and there are some great places along the North East coast of England and South East Scotland such as Beadnell, the Farne islands, Eyemouth and St. Abbs. It is often a welcome change from the usual tropical reef diving and I did my first sea dive here earlier this year. There are also some great places in Northern Scotland such as Scapa Flow, Western Scotland like the Kyle of Lochalsh (both of which I have visited) and Cornwall offers some good coldwater reef diving. Although the water may not always be the crystal-clear blue waters of the Pacific or the Caribbean, it hosts just as much life in just as much variety. There are also lots of wrecks to explore.
If you are thinking of diving the Caribbean because it's closer to home, then try Central America such as Costa Rica and Belize or somewhere like Bonaire amd Curacao because often the most famous places like Jamaica and St. Lucia have reefs that have been spoiled by tourism, the same in parts of Thailand and the Great Barrier Reef.
If you have the money, then I would bag the Maldives while you can as global warming should have them underwater in ten years. A lot of the less known atolls have corals on pristine condition and this tropical paradise has clear waters and a huge quantity and variety of fish and invertabrate life. I'm hoping to go too.
And by the way, after reading this through I've realised it sounds a bit like an advertising sham but I can assure you, I'm juat a 14 year old girl who cares that you make the most of and thoroughly enjoy your holiday. Have fun