Hi Ryan,
As the previous poster mentioned, the weather can be very changeable in this area in the winter months, but in general is quite smooth. In about a dozen cruises to the Mexican Riviera we have been very fortunate and have never experienced rough seas. We just returned from a seven day cruise to Puerto Vallarte on the Ryndam, which is much smaller that the Mercury, and had beautiful weather. We had a maximum wave height of about 5 feet and 15 mph winds, with a barely noticeable roll for about a half day. We have also sailed on the Mercury, and while we never experienced any bad weather, the ship is very stable and you should have a wonderful cruise in these waters. As the previous poster mentioned, it is always wise to have some motion sickness remedy available just in case. You do not want to risk the chance of having a bad experience due to the weather, but you shouldn't worry too much either.
Peter