I've seen beaver working on dams in daylight. This is especially true in the fall. They seem to get into a work frenzy in October.
As Hal said, sundown is your best time.
I'd like to hear about the chainsaw and cutting dams. That is pretty rough on equipment.
Any lessons learned?
I have noticed that when a beaver dam blows a hole at the bottom (not from top to bottom but just a hole in the bottom)it seems as if the beaver cannot fix it.If you break it from top to bottom they patch it in a hurry.
If my observation is always accurate that would mean that a chainsaw cut THROUGH the dam could solve a beaver dam flooding problem.