Holbox is a delightful village and yeah, you can get there in time for dinner if you fly in early enough. All you said was PM?
We did 3 dives in PDC, checked out of the hotel, drove to Valladoldid the wrong way, got lost, and still got to the dock in time to hire a private boat over, was met by a golf cart & driver, unloaded at the hotel, and ate - then caught the 7am boat the next day. If you take the correct road, it should go much easier, depending on what time you arrive in CUN? DO NOT TAKE THE TOLL ROAD SHORTCUT FROM THE AIRPORT! You have to go into the city, then take the free road that goes thru villages and over speed bumps, but it's not bad. Here is a detailed, zoomable map:
http://www.locogringo.com/lgunet/images/maps/roads/masterQR.png
Note 1: There is a lesser road on the map south of Cancun that cuts to 180 Libre. I don't know anything about it.
Note 2: There is a lessor road on the map Valladolid Nuevo to Vicente Guerrero that I have heard is ok and a good way to save miles.
Note 3: After we got lost and turned around at Valladolid, we went north from El Ideal to Chiquilla, in the dark, dodging potholes, and returned that way to 180 Libre back to Cancun. I guess the potholes are still there?
Note 4: Top off your gas tank in Cancun before heading out, and in Chiquilla before parking as the Chiquilla station was not open when we returned.
You can get really good maps at
cancunmap.com-Your information source that include locations of gas stations and more.
We spent 3 nights at Holbox Dream, who set up our snorkel trips, left early on the final day, drove to CUN, turned in our car, changed shirts and flew home ok. You are required to wear a neoprene suit or a life jacket, the latter causing a lot of drag, so we took snorkel vests which worked well. No flash photos, so I shot video. You certainly want your own snorkel, mask, and fins.
All that said, there are Ops that pickup around Cancun which you may find easier, and you may end up swimming with the dominoes in clearer water as some of the herd goes back east sometime in the summer, and many prefer to go out of Isla Mujeres. You'll find more info in threads in the Mexico forum.