We went in January '04. It was quite overcast and very windy the day we went to Tulum. The ride over was bumpy, but not too bad. The ride coming back was EXTREMELY rough -- it was dark by then, and you could see the lights from the horizon bouncing back and forth at about a 45 degree angle. Luckily none of us get seasick, but we did make the comment that if that hadn't bothered us we shouldn't have any problems for the rest of the cruise.
If you know that you're prone to seasickness, make sure you have medicine with you!