Split level pics require a lot of patience. I would suggest to start it a clear lake where the water is relatively flat with no waves. Before you get in the water, make sure the camera in your hand is an underwater camera or a camera with a waterproof housing specifically made for your model camera. Sit the camera on top of the water and lower it slowly until you see the lens one half in the water and one half out of the water. This step, keeping the lens evenly 50/50 in and out of the water will take a lot of practice, but the results are pretty amazing. In this position, click as many shots as you can, it helps to have turtles, fish or people in the water to really make the shot fantastic.
Enjoy and record all your adventures!