Head down to Phuket, Khao Lak, Krabi or Koh Lanta. Flights from Bangkok take less than 1 1/2 hour. If you don't want to fly then go to Pattaya or Koh Chang, but the diving is not as good as you get in the south.
From Phuket you can dive at Phi Phi Islands, King Cruiser Wreck, Shark Point, Anemone Reef, Koh Doc Mai, the Racha Islands and Similan Islands (long transfer time). The marine life is pretty good at most dive sites (except the Racha Islands - might as well go diving at our local dive sites). You'll fly to Phuket airport and have around 45 min - 1 hour transfer time to your hotel (in rush hour much more). There is a lot of tourist in mid-december. I would recommend staying around Kata beach or in the less crowded Rawai beach.
From Khao Lak you can go diving at Similan islands, Koh Bon, Koh Tachai, Richelieu rock and a couple of very nice wrecks a few miles off-shore. These dive sites are amongst the best in Thailand. You'll fly to Phuket airport. Transfer to Khao Lak is around 45 min. Stay in Khao Lak. Ban Niang is another option with much less tourists. Koh Bon and Koh Tachai had a lot of Manta Rays last season, while Richelieu Rock is by many considered the best dive site in Thailand.
From Krabi you can dive at Phi Phi islands, Ao Nang islands, King Cruiser Wreck, Shark Point, Anemone Reef and Koh Haa Yai. Almost similar dive sites as Phuket but Ao Nang islands and Koh Haa Yai really makes the difference. Fly to Krabi airport. Transfer to Ao Nang is around 45 min. Stay in Ao Nang. Lots of good restaurants and small nice bars.
Koh Lanta offers diving at Hin Daeng, Hin Muang, Hin Bida, Phi Phi islands, Shark Point and Koh Haa Yai. I will rate this destination as second to Khao Lak in terms of marine life and corals. Usually a great chance to see Manta Rays around Hin Daeng. Koh Lanta feels less crowded than the other destinations. Fly to Krabi airport. Transfer to Koh Lanta is around 2 hours but can be more. Stay at Long beach and enjoy the fantastic sunset.
The trip to Pattaya takes around 2 hours with car from Bangkok. There are other people on this board, who can tell you more about the dive sites. For Koh Chang it's longer.
The prices varied from destination to destination. Khao Lak is the more expenssive while Phuket are the cheaper one. My recommendation based on the quality of dive sites, marine life, time spending getting to and from the dive sites and what else to do, will be Khao Lak with Koh Lanta as a strong second. You are welcome to PM me for more details.
Michael