Emily, we have been to Xel-Ha several times. We did it like Penny but found that you really don't get enough time at Xel-Ha if you go to Tulum too. I looked at your itenarary and I don't see Xel-Ha as an option through the ship. Not to worry though. Check out their webpage: http://www.xel-ha.com/
It will tell you everything you need to know. I have some pictures too. If I remember correctly, a taxi ride will cost you $10 to downtown (for everyone not just one person) so you can catch the ferry. Just make sure you keep up with time as the ship will not wait for you. Enjoy your cruise!