Help! Anyone know how to make water or a lake on Paint Tool Sai? Please?

It's not about how to do it in SAI. You just have to know how to draw/paint water in general. So to do that you should look at a picture of water and paint what you see. It's okay to use references, it's not cheating and looks a helluva lot better than drawing it from your imagination because your imagination clearly does not know what water looks like.