So...here's the deal. Only some L-series lenses are 'weather and dust resistant' - mostly the newer ones. The way to tell is that sealed lenses have a rubber gasket at the mount ring. Older L lenses lack sealing because Canon hadn't yet included it as a feature. Most newer L lenses are sealed, with some exceptions (e.g. 85L II).
The 100-400mm, and other per-1999 white lenses, are actually partially sealed. They have moderate sealing at the switches and focus/zoom rings, but lack the mount gasket. Note that this info isn't in spec sheets, it came from an email exchange I personally had with Chuck Westfall (Canon's tech guru).
As for water damage, if you see moisture inside, or you know it's been exposed to water and electronic systems stop working, water damage may be the cause.