if you mean the water wheels in otto's swimming pool, then here's how:
At the end of Otto's pool on the right hand side there will be a pathway guarded by some gnorcs, go to the end of that and you will see a water wheel. There should be two things that look like roll bars on the wheel, jump on to the one on the left hand side. then jump on to the other one, and the wheel will start to turn. you rotate round and then jump back on to the next roll bar, by this time a power gauge should have appeared. You need to jump back and forth on the roll bars until the gauge is full, and the pool should be half full. Then you can get to the other side of the pool, go through there and when you get into a clearing go left, destroy the dark gem and choose the pathway on the right. There should be another water wheel, this time with platforms on it, and you can do the same thing as you did with the last one (jump back and forth) until the gauge is full and Otto's pool will be completely full. Go talk to Otto and he'll give you a light gem.
Note: the pathway on the right when you get into the clearing leads to the entrance to sunken ruins, you smash a dark gem somewhere in there and this weird colourful thing will rise out of the water, horn dive that and press Y / triangle, it ought to go into an aerial view and start taking you down.
Hope this helps!