We have been to Cozumel many times, even in January, but somehow have never been during the Carnval. Try this link to get info about the Carnival. It might help.
Cozumel Insider has a lot of info on Cozumel that you will find very useful as well, not just the Carnaval.
Been there - done that:
2005 Holiday, Sensation, Conquest,
2006 Conquest, Celebration, Holiday,
2007 Freedom Grand Med, Holiday
2008 Fantasy & Sensation,
2009 Fantasy, Holiday & Dream Grand Med
2010 Fantasy and B2B Elation
2011 Monarch of the Seas
2012 Booked - Breeze from Barcelona 12 days