We will assume you can at least get to the Cancun Airport - sponsored by Corona (no kidding) - there is a HUGE Corona logo on the tower.
It will take you at least 2 hours (but really closer to 3, and it is always an fun little mexican adventure!) to get to our casa from the time your flight lands depending on your time spent in Immigration, Customs and getting your bags, etc.
If it's your first time here, to get from the airport to the ferry, then I suggest you use Best Day Transfers - IMPORTANT: when you book, just pick any hotel on Isla Mujeres, it does NOT matter! AND, do not book a ferry ticket with Best Day! more on the ferry later, AND book ONLY the "Shared" shuttle because even though there is a chance you will have to go thru the hotel zone, that has not happened to us in 10 years!
You should make your Best Day reservations in advance - Karen & Jimbo say 'bueno'! (they need 24 hours notice, no exceptions). By the way, you can take a taxi from the airport, but it will cost $65.00 usd one way, Karen & Jimbo says 'no bueno'.
Go to their website to book a reservation or give them a call at 800-593-6259. If you decide to call them they usually speak pretty good English. as of September 2014, their rate is 28.00 usd per person round trip from the airport to the ferry at Gran Puerto at Puerto Juarez. their website is very thorough and you can book a ticket on the Ultramar ferry from Puerto Jurarez (just north of Cancun) to Isla Mujeres...but do NOT buy your ferry ticket here - just because.
When you exit Customs, there will be a bunch of folks trying to get your attention to sell you a Time Share - just put your head down go straight ahead and GET OUTSIDE and do not stop no matter what - if you stop, you will regret it - and they are VERY good at getting you to stop - i know because i knew this ahead of time and they still got me to stop.
When you get outside, it might feel like you are in Mexico, but you are not yet there - patience, you will get to Mexico soon - remember you are still in Cancun which has been annexed the the US.
Then just look for the Best Day uniformed folks. always nice and always there - ALWAYS - 100% of the time - even if you are late, they are there - even if you are really, really late, they are still there. Always!