Typically a harbour doesn’t have one razor-thin path of traversable water. Instead there’s a clear pathway that you shouldn’t stray too far away from. The Moire system will give steering advice even if you’re in a safe area, but slightly off the main ‘beam’.

You could fix this issue by just having a second lamp (or two grills in front of the same lamp). If the two arrows point in opposite directions, you're safe. If they point the same direction, you're not.