Waterways, even through private land, are held in the public trust. That includes the bank up to the high water line.
True, although Wisconsin holds "public access" rights extend 3' above the high water line for all lakes and streams. If you own all the land surrounding a small lake you can legally post the approaches against trespassing and keep people out. If your "landlocked" lake has a stream either entering or exiting your lake, and the stream flows under a public road, a person can legally start at the road and walk the bank all the way back to your lake and all the way around it.
I have never found out if such "public waterways" were deducted from the acreage specified on your deed for real estate taxes. I have a 24' easement along a 380' edge of my lot for a county road and I wonder about that also. The lot description on the tax bill matches the deed's "meets and bounds" description exactly, including the easement but doesn't make it clear if that is included in the area calculated for tax purposes.
Regards,
GtG