Thanks Mark, Mike and Karl
Answering Karl's question, is more a matter of observation than technique...
After all is the river where I have lived nearby all my life, and I am perfectly aware of the colors that it may be... sometimes it can be deep blue (high tide, depending on the season!).
The technique used is watercolor paper, acrylic paints while the paper is wet, and mixing it according to a color photo or a memory (this was the case). 
If you see my MOD's and Features, you'll notice that all my displays have different colors according to the place where it is supposed to be.
This is a great compliment... Thank you gentleman!
Cheers
Rui