It's "Chan" as in "cha cha" with an "n'" on the end
"Kuh" as in "Duh"
"Knob" as in door knob
Hot tip: Bypass Chankanaab and go a few Km. further to the Palancar Beach. No entry fee, and you can SEE the Palancar Reef from the shore. For $30 you get two hours of snorkeling from a boat AND then you have unlimited time to play with their water toys or just laze about in a hammock in the shade.
A taxi will be $15 each way (possibly negotiable) but for $45 you can rent a Geo Tracker right on the pier and be on your own.
Chankanaab is like the Jersey Shore on the Fourth of July weekend. You couldn't get me there on a bad bet.