Other than Cozumel, where would you recommend to visit that has great beaches and great diving? Trying to plan a trip for my 30th birthday. Thanks.

At the risk of sounding like a zealot, Virgin Islands! Look into it. Your're 30... it's great grownup fun.

Cayman Islands, and we've been to Cozumel and USVI.

In addition to the USVI (and BVI) and Cayman Islands, there's always the Turks & Caicos. Beautiful beaches and excellent diving.

great, thanks for the recs. I'll check into these islands.

Turks and Caicos and St. Croix are two excellent choices for beach and dive. Enjoy.

I second, (or third) Turks and Caicos.

You might look into some of the outislands of the Bahamas. Harbour Island has one of the largest reefs in the world just off shore, and there are 2 dive shops on island. Valentine's is one of the best known in the region and I know there is a website.

You might try:
www.myharbourisland.com
www.rockhousebahamas.com
www.romorabay.com (Romora also has a dive shop which is Fox Dive)

Have a great time wherever you end up going.

The only other places than Coz I know that have great beaches AND diving:

Provo, TCI
Isla Mujeres and Akumal, Mexico (beaches not as good)
Some sites in the USVI's (Virgin Gorda)

You might want to check out Curacao.
The island has great beaches and wonderfull diving spots.

If you want to take a look for yourself you can check out the following links:

Beaches:
http://www.curacao.com/visitors/beaches.html

Dive spots:
http://www.curacao-aktief.com/content.php?id=3

You might want to copy paste them in your adress bar, it would be much easier