You can avoid an expensive car rental (and the very considerable time it takes to do so) in Cancun for a trip to Akumal, by taking a cab there and then renting a golf cart. Just be very careful about renting one from Akumal Guide. Make them give you a copy of the contract, which includes a deposit of $50.00 USF. They wanted to keep my entire deposit since I left the grounds (unintentionally, and they kept the only copy stating it was a no no). They eventually relented and gave me half, after a hell of an argument. They also "encourage" having the transaction done in cash by adding 5% to the charge to "cover" the credit card fee. This of course is a "no no" from the credit card issuers standpoint; but that part Akumal Guide chooses to ignore.
There are others to choose from if you want to rent one that are more scrupulous.