December is peak season here and room rates will increase from October onwards but depending on your accomodatin requirements most dive centres will offer you a package deal (at least we do) for your hotel / diving / transfers so you can give them your requirements and see what they can offer. I have recently certified two Singaporean Advanced Divers that even here in Low season were thrilled and impressed with the training / diving conditions and cost of diving here in Phuket.
Good Luck
Good Luck