There used to be a time you needed to make sure you had cash money before coming to Koh Tao, these days are long gone now and money is readily available on the island.
Koh Tao has four full service banks - Thanachart Bank, Siam Commercial Bank and Krung Si Bank are open from 9.00am 3.30pm Mondays to Fridays. Siam Commercial money exchange is also open on a weekend. Bangkok Bank is open seven days a week from 9.30 am until 16.00 pm. All four banks are located in the main pier town of Mae Haad.
There are also numerous ATM's and exchange booths around the island so your money is easily accessible on Koh Tao. In case you would like to get money from overseas, Western Union outfits can be found here as well.