Truck sat out during a three day snow, followed by warm weather and the snow melted (was at a conference, so the truck was parked the while time).
A little bit of intrusion from the passenger side just behind the wheel well, a little from in front of the roller tonneau case, and a little from the tailgate seal with the cover.
Pretty much the same as running through a car wash. Probably less than a cup of water total.
If I have stuff in there that has to stay absolutely dry, then I'll probably put something in to make a little space between the bed and the items. Maybe the bed mat, if it keeps a gap on the edges so the water can run under the mat.