It’s not public, but Everts Resort is within 2 miles of the dam and it’s the closest.
For 7 bucks you can leave your rig and have very little to worry about. Good up to date fishing info as well. There’s other places to fish besides the dam. Check in with Steve or Kari.
I’ve been leaving my rig there for 7 years and never thought about vandalism while out until the wee hours of the morning.
There is the DNR ramp in the WI back channel that’s free…and you get what you pay for.