Based on recommendations here, we recently rented from Easyway, very economical if you drive standard. The Nissan Tsuru was on a par with our local Rent-a-Wreck franchise: serviceable, easy on fuel, got the job done. I'd rent from them again.
The reef diving off Akumal was better than posters here had led me to expect, with interesting geography, though honestly not a lot of colour and never that aquarium effect. Appreciated the lack of current after Cozumel. What made it an outstandingly relaxing, fun week was the expertise and good cheer of Tim and Katrin from Blue Experience Diving. What a wonderful outfit. They use boats from Akumal Dive Center, as do Natalie and Ivan. We shared a boat with the latter a couple of times and they seemed good guides too.