There are a couple of twists on how to get there -
The flight is a problem - hard to get seats, doesn't leave when it says it will, and overweight charges - all true.
One big advantage of taking the flight (especially if you arrived in the Charter terminal in Cancun) is that you get to jump the Customs line up. My family spent 2 1/2 hours standing in line just to get through customs. What an infuriating way to start a holiday.
But, I found later, that some people went through the "shortcut" line said to be just for Mexican residents, and AeroCozumel passengers - without a ticket on AeroCozumel. Next time, I will try that.
Then, get yourself to the Playa del Carmen Ferry terminal. There are lots of ways - all about the same price. I have taken a booked "limo" - Suburban - for about $70, for 6 people - or you can just take a taxi - but bargain the price first. Don't ask how much after you get there. Should be about $30-35 for a small taxi seating 4 people. The bus is even cheaper, and not much slower. Driving time is about 40 minutes or so.
None of these will take you right to the boat - you have to walk a bit - but not too far. Cars and taxis will usually drop you off on the south side at a hotel turnaround - maybe a 3 minute walk - the bus station is just kitty-corner across the Zocalo or city square - maybe 4 minutes walk. The ferry is $8 each way, and takes 40 minutes. They will check your large bags for you and put them on the boat. You can buy drinks or even a cheap t-shirt on the boat. Then, a cab to your hotel. The further ones away are in the Iberostar area - they might charge you more, but the actual fare for 2 people is 150 pesos - $15. Rental cars are unusually expensive on Cozumel.
It takes around 4 hours all told - depending on customs, and the timing of the ferry. It runs every hour, or every second hour, depending - check the schedule on the web.
Have fun.
John