Given that it's a two hour boat ride to the nearest land, you may want to be aware of the healthcare situation is on Koh Tao:
The island does not have a full service hospital of its own. The nearest one is on Samui. However, there are several clinics where you can get medical attention, including emergency assistance. There is a hyperbaric chamber at Physicians Clinic in Sairee Beach. They have a fully qualified doctor and nurse on staff, and have facilities for overnight admissions.
In Mae Haad, Mae Haad Clinic is open 24 hours.