This year, due to the U.S. and world economy, and the travel slowdown, it's harder to say what the occupancy rates will be in early December. Hotels are still heavily booked for Christmas but cancellations are coming in and people are not completing their deposits. San Pedro is always the most heavily booked area in Belize -- outside of San Pedro there should be rooms in most price categories.
In early December you'll have an easier time of finding rooms, but I would encourage you to book ahead if possible.
Canadian currency is not accepted routinely in Belize. You can exchange it at most banks, but it's far smarter to travel with U.S. currency which is accepted everywhere and used interchangeably (at 2 BZ to 1 US) everywhere.
Belize First Magazine