There's no direct bus to Akumal from Cancun. Yoú would have to take the bus to Playa then either take a Mayab (second class bus) that is heading to Tulum and ask the driver to stop on the highway, or take a colectivo. Reading a bit about the trip, some say 3 or 4 hours from Cancun!! Driving takes less than an hour and a half, those buses and colectivos must stop every two seconds. Add the time getting into Akumal (taxi) and that's a long haul.
We have always used Ace rent a car in Playa! They have 2 locations in Playa! On 307 & 5th ave! Nice people, clean cars & a good deal! www.acerentacar.com As you know buses & collectivos will stop at the 307/Akumal exit! You still have about a mile to the Arch! If you drive watch the exit! Very confusing! Fun town with lots to do & see!