The meaning of "coastline" breaks down far before you reach quantum levels. I challenge you to go to the beach and pick a square inch where the "coastline" splits it half and half, this part England, that part the sea.
This is also ignoring the time component, where individual fluctuations in water will change the measurement, tides will as well on a daily and monthly cycle, and as years go by erosion and global climate change will shrink the land and grow the sea.