Wooden boats normally had a scribe line put there by the builder to mark the waterline, as a guide for painting. It would be about .5mm deep, and 1mm across.
Many years ago, someone made a similar one on an aluminium boat.
Salt corrosion and stress (flexing, I guess) did their work remarkably quickly to expand the scribe line into a deep, pitted score along the hull with hairline cracks running off it, and the boat was written off before it sank with all hands and the ship's cat.