We can usually pay for one persons diving by flying into Cancun, doing the bag drag, and ferry to Cozumel. We Save 200 to 400 on round trip for 2. The bag drag is really only about a block and a half. Exit the airport doors, wind around to the right, buy your ticket at the Ado podium outside and enjoy the ride to PDC. The busses are new and air conditioned. From the PDC bus terminal...walk a short block to the sea, turn right and go 1/2 a block and buy your ticket. The ferrys are large, you can't miss them.
There are are also multiple flights into cancun so scheduling is usually easier. In the likely event that American Airlines screws up your flight into Cozumel, (they have botched 3 out of 4 flights this year for us) you can usually get into Cancun and not lose a day of diving. Good to know and file away for future reference.
There is is a webpage that shows how to do the the bag drag, we bring all our own equipment except for tanks and weights and I'm the Sherpa for me and my wife. I'm fat, over 40, and not in the shape I was 20 years ago and I don't mind it at all. Its really not that big of a deal.
Cancun Airport to Playa del Carmen/Cozumel by ADO Bus
as far as afternoon conditions, I've never seen it get rough until after dark unless something is blowing in. If your flight gets in late, the only bad ferry trip I've had was leaving at 8 or 9 pm, I've been on smoother roller coasters and even the locals were using barf bags. There is always a risk that the seas will be rough, winds high, and the port will be closed. It's just that time of year. Aldora has a larger boat may be able to dive when other cant. When you book your dives, make sure you ask about port closure contingencies. Even when they close the port for the early morning, they may open it up by lunch.
I enjoy the trip, but if you can fly directly into Cozumel for about the same price, that's the way to go. It gives you back 4 hours of your vacation and eliminates some of the travel stress. But Like most of the people on this site, diving in Coz is where we would rather be. Knowing the bag drag usually will save you some money, and may save a day of diving if the airlines can't get you to Cozumel for any number of reasons.
Safe travels, please drink a Bohemia Obscura for me at The Thirsty Cougar!!
jay