The small Tha Khao waterfall is certainly not very impressive either by its size or its flow but surrounded by forest and little known and therefore almost always deserted, it is a pleasant little fresh and bucolic hidden gem of Koh Yao Noi.
If you can wade there, it's not a waterfall for swimming, but since it's close to the main road, it's a nice little trail to reach it and a nice hideaway.