Scuba diving in Thailand can be divided very easily between the east and west depending on when you go due to the monsoons. 

The good news is that, when one side is affected, the other isn't with the west coast (Andaman sea) diving season from November to April and February to May is the calmest time of year on the east side (Gulf of Thailand).

Thailand West (Andaman sea) - if diversity is what you're looking for, this is the best side for reefs and fish life which can include both leopard sharks and manta rays.  Most dive facilities are on Phuket Island.  Dive operators from Phuket, who arrange trips to the Similans (a national park), shuttle their customers to Tap Lamu pier in Khao Lak. The trip to Khao Lak from Phuket takes 60-90 minutes depending on where you are staying.  The trip to the Similans from Khao Lak is 55 kms by sea.

Thailand East (South China sea) - Koh Tao is famed for offering the best dive sites and the resort of Pataya (not the most spectacular diving destination) has the wreck of the HTMS Kut, an ex-Thai Royal navy vessel. However, Koh Tao is over-dived and damaged - it's one of the biggest cerifying places in South-East Asia.  Thousands of new divers do their Open Water certificate here, year after year, so the reefs have been damaged by new divers learning to control their buoyancy.

Diving from Koh Phangan prevents divers diving on overcrowded dive sites and more chances on diving on the more remote places (for example Sail Rock, one of the best dive site in the Gulf of Thailand and Ang Thong Marine Park)

However, trips from Koh Phangan do go to Koh Tao as well, so you might like to check your dive destination before booking.  Many dive centers on Koh Tao head out at the same times and mostly to the same sites - 10 boats on 1 site is not uncommon here, with big boats taking up to 60 divers. It would be advisable to do your research throughly with the local dive centers before you travel. 

• A yellow fever vaccination cerificate is required from travellers over 1 year of age coming from infected areas
• Malaria risk exists throughout the year in the whole country. Prevention recommended.