Hi Fernie. Entire guidebooks have been written to answer the questions you pose, but very briefly:
• The best time for you to come to Thailand is when it's cold and snowy in Toronto!
• The best area to visit since you're coming halfway around the world is the Andaman Sea coast of the country.
• Phuket is the hub of Andaman Sea diving and the location of the main airport in the region, but you can also base yourself at some of the other islands off the coast or at some of the coastal cities.
• I recommend a liveaboard trip to the Simian and Surin islands national parks, and of course you'd stay on the boat for a trip like that.
• Nitrox is widely available here and the average price for Nitrox diving is about THB 250/tank. Some operators charge a little less, others a little more.
• Thailand is a country of rich cultural heritage and natural beauty. You can travel the country to sample the various regions or simply immerse yourself in one area.
• We use Thai Baht here, but ATM machines are available, so you can cash money as you need it, or you can use credit cards. I wouldn't bring stacks of currency along, or even traveler's cheques.