I don't really know where Costa Maya is. Is that the cruise port? I'd probably fly into Cancun and take a bus.

If you're trying to do this from the US, be sure and check the laws, they're very restrictive about letting people on/off other than at the point where the cruise starts.

Cancun.

Cozumel is a seperate island surrounded on all sides by water. No way to get on or off the island unless by plane or ferry to Playa del Carmen.

I googled and found the information below; since it's 100 miles south of Playa which is ~45 minutes south of Cancun but requires (as above) a ferry trip to the mainland so either would work.
___________________

Costa Maya, the newest port in the Western Caribbean, may be located on a peninsula but in effect it feels more like a private island created from scratch expressly for cruisers. Costa Maya is located on the Caribbean Sea approximately 100 miles south of Playa del Carmen and quite close to the border of Belize.

Costa Maya is what you'd expect if, say, Disney World decided to create its own private island in Mexico: a man-made tourism village with bars, restaurants, shops and pools at the ready. The faux village itself was created solely to woo cruise passengers and it has a myriad of facilities -- pools, restaurants, bars, shops and a small beach (though it's too rocky to swim).

I think the closest town is Mahahual. Friends stayed there a few years ago. It's about 3km from the cruise port.

The closest town is Mahahual but getting there is not the easiest proposition. Here are some choices:

1. Rent a car and drive yourself. There will most likely be a car rental in Mahahual also and then you would take a taxi to the cruise dock. Additional cost to take the car one way and drop fee.

2. Take the ADO bus. They did have some service to Mahahual but I'm not sure if they still do. Other nearest city would be the capital, Chetumal. Then take a taxi/bus to Mahahual or the dock.

3. Fly on Interjet which has service between Cancun and Chetumal. Then take a taxi/bus to Mahahual or the cruise dock.

The trip is about 4 hours from Cancun. The taxi from Mahahual to the cruise dock is about 5USD.

Good luck.