The ocean diving in Akumal isn't like Cozumel. Current is very mild. No deep walls. Nothing absolutely spectacular. The reefs are pretty healthy, nice corals, fans etc., lots of rays, turtles, huge eals and baraccuda, some sharks, the usual assortment of tropicals. The dives were nice, but not spectacular. I'd say much better then the Bahamas, but no where near Cayman.
There are several cenotes to dive close to Akumal. Since the weather isn't a factor with the cenotes, you can dive them on days that it's raining or the ocean is too rough.
Akumal is a really nice area. Very laid back, very relaxed and not a whole lot going on. There are about 7 restaurants and 2 bars and a handful of shops. Lots of affordable condos to choose from, and also nice villas. We've stayed at Vista del Mar condos and dove with Akumal Dive Adventures.
We've liked Akumal because it's quiet and laid back, and we like the combination of descent ocean dives and the cenotes. It's also about 25 min to either Playa del Carmen or Tulum if you want more restaurants, nightlife or other topside activity.