Recently, I was cruising on the Danube river from Bratislava to Vienna and we were encountering these small green rocket-shaped objects floating in the water roughly after each kilometer. They were not staying still but also sailing in the same direction as our boat but slower.
What are these UFOs (Unidentified Floating Objects)?
Those are moored buoys marking the fairway in the middle of the river, where the water is deep enough for safe sailing. A green triangle marks the starboard side of the fairway (left side, as seen in the direction of water flow) and on the other side of the river, you would have seen red buoys with a square plate on top, marking the port side of the fairway.
Your impression that the buoys are moving is just an optical illusion due to the water movement.
