land owner keeping water open
Any safety hazard presented by this would be minimal. When winter actually comes, the wind will have little to no effect on where and how big the open water is. My suspicion is that the area affected represents an extremely small percentage of the lake surface, so close to shore as to have little to no impact on angling activity. My suspicion is that the whole thing looked a lot worse than it actually is because of the recent warm weather and dicey ice conditions. If i owned land on a lake in waukesha county, i would be tempted to do this as well, it makes one less place for people to leave empty beer cans and fast food wrappers.
Looks like aerator guy's pressed his luck and betrayed your kindness. I'd go ahead and take his dock away. Let us know how it goes.
My guess is dnr is already full aware of it and would guess it's for winter kill like was mentioned. After last winter sure it's more common in lot of lakes this year.