I had gotten some requests for a tutorial on MyPaint, an open-source program which I often use these days in combination with GIMP.
This resulted in this MyPaint + GIMP tutorial, which coincidentally also tries to teach some ideas about painting calm water ...
(click to go there, it is hosted on gimptalk.com like all my tutorials)
Enjoy, and hope it's of any use!
~~~If you are interested in the art tutorials I have done, you can find them all here: [link] .