I am not professional but I dove in Thailand many times over different years.
I think base from Phuket or Krabi, you can go North and South of either city, nothing will go wrong !

If you get a casual vacation to Thailand Phuket island, pick up a dive trip with whoever that will take you to pi pi island, Ko Bida, Bida North. YOu will have nice dive. You can also go south on another day to Racha Yai, Racha Noi or North to Similian island. These are fantastic places.

If you go to Krabi, you can dive from Krabi or go south to Ko Lanta. Lanta island is my favorite, it's basic, not bars, and just basic ! It's really South of Thailland, take a trip down to Hin Daeng, Hin Muang, they are the beautiful dive site not affected by tsunami.

Anyway,I think I type too much. I love Thailand, any question, let me know !