annlaur
Contributor
So, I'd been hearing and reading in different places (including on WikiTravel) that it was not allowed to take a rental car on the Calica-Cozumel ferry. Being a true journalist, I didn't check my facts and just repeated that urban legend to whomever asked me about it… 
Today a friend of mine told me he was planning on crossing from the mainland to the island with a rental car, as the rental company had assured him it was OK to do so. So I contacted Transbordadores del Caribe (who operates the ferry) on their Facebook page and asked.
Turns out IT IS ALLOWED TO TAKE THE CAR FERRY WITH A RENTAL, as long you can show the rental contract and paperwork upon boarding. Transbordadores del Caribe also suggested to let the rental agency know of your intentions beforehand, just to make sure it's OK with them too.
Now that does not necessarily make the car ferry a desirable option for those planning on a few days' visit. There are few departures a day, the boat may not run if sea conditions are bad, it may be full so it is advised to get in line at least an hour before boarding time, tickets can't be bought in advance…
For more info, here is Transbordadores del Caribe's website : Bienvenidos a Transbordadores del Caribe | Cozumel, México
For the more accurate schedule, check their Facebook page, where they post the updated schedule for the following day : Transbordadores del Caribe | Facebook

Today a friend of mine told me he was planning on crossing from the mainland to the island with a rental car, as the rental company had assured him it was OK to do so. So I contacted Transbordadores del Caribe (who operates the ferry) on their Facebook page and asked.
Turns out IT IS ALLOWED TO TAKE THE CAR FERRY WITH A RENTAL, as long you can show the rental contract and paperwork upon boarding. Transbordadores del Caribe also suggested to let the rental agency know of your intentions beforehand, just to make sure it's OK with them too.
Now that does not necessarily make the car ferry a desirable option for those planning on a few days' visit. There are few departures a day, the boat may not run if sea conditions are bad, it may be full so it is advised to get in line at least an hour before boarding time, tickets can't be bought in advance…
For more info, here is Transbordadores del Caribe's website : Bienvenidos a Transbordadores del Caribe | Cozumel, México
For the more accurate schedule, check their Facebook page, where they post the updated schedule for the following day : Transbordadores del Caribe | Facebook