What does Boxing Day have to do with boxing?
1708, "distinctive mark on paper," from water (n.1) + mark (n.1). Cf. German wassermarke. Not produced by water, but probably so called because it looks like a wet spot. The verb is recorded from 1866.
design produced by creating a variation in the thickness of paper fibre during the wet-paper phase of papermaking. This design is clearly visible when the paper is held up to a light source