I made this in a few hours this weekend. The sand and water are procedurals. The sky was from a photograph I took with my digi-cam (world setting matched for water render). I made two renders and then combined and added text in PS. I also add the seagull in PS (it was in the sky picture I used). My question, would this be considered pure? How could I make it so (would putting the alpha bird texture on a plane and then rerendering work)?
Good job with the effect of the castle disappearing over time 
IMHO, the whole “pure” idea is a complete joke - unless you are submitting to a contest with rules about things like that. I know of a guy who submitted an award-winning image to CGTalk that contained a “3D” character who was hand-drawn in Photoshop. You couldn’t tell at all, and no one would have cared, because the image looked so good.
Shouldn’t the water look a little higher in the image next to it. Like the change of the tides?
the tide has come and gone back out. I also purposely didn’t show where the water and sand join. It was a quickie for the WC so I kept things simple…