Do like I once did, way back when: white-glue a rock to the top of the screw. Just find a small rock, with one flat side, and glue it on. Many shallow streams have rocks in them, and if you ever need to access the screw, it comes off easily.
In my case, the screw was on land, so it was even easier to hide under a rock. A little dirt around it, and it looked like it had been there forever.