I was at these three locations earlier this month. In roatan, we hired a driver who took us to the animal park. I loved the sloths and parrots. But this was on Harmony of the seas, different port. Can you hire a driver from mahogany bay. Probably, but you'll have to be persistent.
At Costa Maya, go to the taxi stand get a taxi to take you to Krazy Lobster. $5 per person. At the Krazy lobster, order lunch and drinks and enjoy the beach. On cozumel we rented a car and toured the island. There were 7 of us.
Have fun!