Health should always be a factor in your preparations when visiting S.E Asia.
There are no major diseases in Thailand and vaccinations are not necessary, all though the risk of malaria or dengue fever is very small, it is recommended to pack mosquito spray. Don't worry if you forget, you can buy this everywhere in Thailand.
The main concerns are the change of diet and more importantly the standard of the water. Most diseases are transmitted through this.
Don't worry about the ice served in your drinks, all ice is made from purified water but like in any tropical country, cooked food is less risky and use bottled water to brush your teeth.
For divers, the main concern is dehydration, which can be easily avoided with a little common sense. Make sure you drink plenty of water. Don't think that if you're not thirsty you are OK. It is recommended that you drink at least 3-5 liters of drinking water everyday and remove your wetsuit when not diving. Remember, wearing a 3 millimeter black rubber suit in the tropics, has a tendency to make the wearer dehydrated.
Koh Tao has 2 clinics with qualified doctors and nurses, plus numerous First Aid stations that can treat small injuries, sun burn etc.
We recommend that you have a travel insurance, so that your diving holiday remains a happy one.