When you arrive at the ferry, buy your ferry ticket one way...no need to buy a round trip ticket, there is no savings and no reason to have something to remember where you put it, 'cause after a few hours you will no longer care where it is...then just buy your ferry ticket when you leave and practice a little spanish with the boletera.
At 160 pesos each way (around 9 bucks usd) as of June 2017, it is an unbelievable bargain!
You can pay in dollars if you need to...remember, you are not yet in Mexico.
You are getting VERY close...and you can feel it.
Hopefully, you do not get on the ferry right away...it will help you to slllllloooooowwww ddddddooooowwwnnnnn...soon, time will no longer matter...it will seem like a silly concept very soon.
And when you get on the ferry: RIDE ON TOP! if you have not yet been to la isla, then you won't believe what a great ride it is...sometimes the ride up top comes complete with seriously cheesy musicians that are super cool to listen to even if they are not perfect...and a cerveza is a MUST...even at 8:30 in the morning.
Aide bar: the ferry runs every 30 minutes from Puerto Juerez to Isla Mujeres starting at 6:00 am...or maybe 5:30...or maybe whenever they get out of bed and get going, ya never know...until 9:30 pm when the ferry will run Puerto Juarez to la isla every hour at 9:30, 10:30 and the last ride is at 11:30 pm...if you miss it, you just missed one night in paradise...so don't miss it.
When you get to the other side...bienvenidos a Isla Mujeres...NOW you are in MEXICO!!! and you have officially escaped Cancun and the US (or the planet Canada)...and it is fun, and inexpensive to let the guys take your bags for you...20 pesos or a couple bucks per bag...those guys will get your bags to the taxi for you while you go get some pesos...the taxi stand is next to the ferry dock, you cannot miss it...and taxi is the best way to get to Casa Roca Caribe.