As CajunDiva says, December isn't a bad time to dive Roatan, and I'll add that it's not my favorite. Lots of critters are alive and moving in the July water column.
The plus to diving the South side at that time of year is that it is on the sheltered side of the island from high winds and lashing rain storms. It is on the storm-sheltered side all the time, but this becomes a consideration during the months of Sept>March as tropical storms may pass close-by to the North, or as "Northers" descend from North America in the colder months.
If it is raining, the shore dive that CCV is well known for isn't nearly as much fun- the critters don't like fresh water run-off and the colder water, even by a few degrees, that rain water brings. I would go out on the boats more often in those conditions. A Goretex unlined parka would be a nice accessory.
Visibility doesn't return as fast to the South side, but many old timers see that as no big deal. The creatures that you are finding and looking for are the macro variety- so you are spotting them in the 8' and closer range, anyway. On the North side, you miss that nursery zone and tend to see larger fish and crustaceans- they seem less affected by water changes of salinity and temperature. These creatures blend in well with the viz that seems to return more quickly on the North side.
In these conditions, it can really pay-off to follow the DM and get pointers on how to find critters. So many people come to dive at FIBR ask about if they have to follow the DM's, at CCV most visitors follow the DM's attentively. There really isn't much to find or see on your own, unless you've learned their tricks, which they will share.
The Full Moon is on or about December 21st in 2010, and that might be an ideal week to be there. Local seafarers find that this is a time of calmer seas- might be fable, but everything else that they have told me about local weather has proven to be true.
