I use a skin year-round, but at this time of the year can occasionally be a bit cool. Doubling up the way you describe is good, or if you want to bring a single suit a full length 3mm is probably ideal.

My husband wears a t-shirt and shorts when we go in November - but I wear a 3 mm, and I am more apt to be cold. Depends on your comfort, but the water is warm - 80s. We did the Turneffe dives, and the water was 84 at 80 feet. Have fun - it won't be difficult!

Here's a link to our pics to get you psyched for the trip: http://www.angrypig.com/belize05/