I overlaid a photo texture of water (from cgtextures), used transform tools to distort it how I wanted it, made color adjustments, and painted over most of it until it meshed with the rest of the image. It's a technique often used in professional concept art. A useful shortcut, but still takes some finesse to get it to look natural.
It's a technique I've heard people use quite a lot, sounds like a great way to save time and practice whatever it is you're painting over. Is the photo still visible in the final image then or do you use it as a guide and paint on a new layer? I'd love to try this out
The phototexture is still in there somewhere, but the vast majority of it has been painted over. For me, it's not about practice, it's about saving time—I could use the photo as reference only and paint it all from scratch, but there's really no point when I can get the same result a faster way.