Ton Ao Yon Waterfall is a rather hidden swimming hole around 3.6 km east of the Kao Khad View Point.
Getting to the top of this waterfall is quite difficult because of sharp rocks and rather confusing pathways, so you need good shoes to protect yourself.
Ton Ao Yon Waterfall has several drop pools that are big enough to bathe in. Since many people skip this spot in favour of Phuket's more popular waterfalls, you'll probably have the entire area to yourself.