Don't be put off by what you read it one book, as you will see from the numerous posts in this thread and others, there is great diving in Cozumel for all levels of experience. As with any trip, just tell the dive operator your diving experience and they will ensure they visit the most appropriate sites. While there is basically always a current, that's one of the reasons the diving is so good, it varies significantly in strength and you just drift with it and enjoy the scenery. We use Carribean Divers when we're down there, a great bunch, know what they're doing, safe and make sure everyone has fun. We'll be down there early August for a week and really looking forward to it. Cozumel is a great dive destination, good diving, good people, good food, plenty of fun to be had when the diving's done. Go and enjoy it.
By the way, I agree with Lars, get the course finished before you go, that way you all get to go out and just enjoy the diving rather than one person having to complete "lessons" first.