I have a full winter cover for my boat. The high point is at the helmsman seat on the upper deck and from there it slopes down to the rear rail and then down to a cinching strap at the level of the swim grid.
The problem is right at the rear rail. The fabric sags a bit, the water collects a bit, which cause it to sag some more, so it collects more water etc. It very quickly collects a huge amount of water, probably approaching 100 lbs. (Say the equivalent of a 20 liter pail on each side which would be 88 lbs.)