Just got back from Aruba on 5-17. As they say....It's one happy Island. There are alot of sites out there for information about the island, so I"ll just stick to the info on the diving.
We (boyfriend & I) went on 5 dives around the Island. We ended up diving with Pelican Adventures, which is located on the pier between the Holiday Inn & Playa Linda hotels. We did look at Red Sail and Unique dive operations but their schedules didn't quite fit with ours. The guys as Pelican were all great. I had sprain my ankle the day before our first dive and was a little concern if I would be able to keep up. Not only did they help me get in & out of the water (without hurting my ankle more), continued to make sure I was OK but actually gave me a tow now and again. The most divers we had on a trip was 7. I think that is unusal. It seems a lot of the dive operators cater to the cruise lines. So if there is a cruise in town, it could be more crowded (fridays are big for that). However, I believe the end of cruise season in May.
We dove the South side of the island - Jane See Wreck and the reef near by. Nice little wreck with the biggest angel fishes I've ever seen. The next day we did the Antilla Wreck during the day and at night. This is a highlight. For a day dive, get out there early. This is a popular site so you get a ton of divers (from all the dive places) and a lot of snorklers. It's a big wreck with lots of growth and sealife. If you do a night dive, definately do the Antilla. We were the only boat out there and only 5 divers on the boat. Beautiful. Eels galore. The next day we dove the Debbie II & blue reef. Debbie has only been down for about 10 years. Not a lot of growth but it's getting there.
The other places I've heard that are good - Gino's Choice, Mangel Halto Reef & Sonesta Airplanes (there are 2).
All in all, warm water, good visiblity, and fun easy dives. I would not call Aruba a dive destination. However, it provides a good mix for land and water activies. I'm sure you had better diving in Cozemel, but I found Aruba very relaxing and a great place to visit. I would definately go back
If you need more info, just let me know.