Like many of the other posts, I would like to chime in on my experiences with Half Moon Bay and the diving there. I stayed at Vista del Mar for three or so days, and went diving with the folks at Akumal Dive Adventures. I had been living in Akumal for three months at the time, so I was pretty familiar with the area.
First, the distance to town is a VERY long walk in the heat, not just 15 minutes, more like 30, and many times I simply hitched a ride with the newspaper delivery truck to get into town. In addition, there are NO lights so if you dare to walk the road after dusk, beware. Bring a flashlight. A BIG one.
Second, I was robbed while a guest at Vista del Mar, and they chose not to do anything about it. The security personnel didn't even believe me that my things were stolen. I speak very good Spanish, so I can't imagine it was a communication problem.
However, I was very pleased with the dive shop. The dives were all great, and the divemaster catered the dives to the preferences of the boat. However, three people went into the water without having their air turned on, which I consider to be both the diver's fault for not double checking but also the assistant could have been more on the ball.
Further, if you want to do any swimming or snorkeling you can't really do it on half moon bay (it's way too rocky and slippery you could get really hurt) so you have to take the walk into town to do any real swimming.
Overall, I would not stay on that end of Akumal again. I had a much better time staying at Hotel Akumal Caribe for the same price.
Sorry for the length, but I wanted to iterate my experiences. Thanks, Jessixa