How did you get into the cave? I can't pass through the waterfall and can't find a hole anywhere.
EDIT: Found a way in by standing in the water at the top of the waterfall and then there is a gap between the waterfall "wall" and the rock where one can fall down into the cave. But i can't manage to get out the same way i came in.
EDIT2: When facing the left part of the waterfall in the cave there are two exits that go down to the lake below the waterfall. If you stand right against the waterfall in one of these exits and look up, you can see there is a big gap between the rock and the waterfall wall, so build a ladder there using Conveyor Poles and you'll manage to climb out of the cave.