At the very beginning of Chapter 3, Lilli will be standing next to a red and white row boat. Get in the boat and have Lilli row all the way from where you start (right side) to the waterfall on the far left side of this area. Once at the waterfall, row Lilli all the way back to the right where you started.
Lilli must make 5 "full" trips for the achievement to unlock. One trip = all the way to the left (waterfall) and back to the far right where Lili first found the boat.