Unfortunately, just as you had written, this is not an easy subject to research without putting in phone calls to people. ACoE usually controls most waterways in the U.S. according one LARGE map I found. Granted this was just a map and didn't say anything in reference to weapons!
I know, that to our (gun owner's) dismay, it's illegal to have weapons on ACoE waterways or "their" oversight properties. But all of this might soon be changing, granted that soon in political circles could mean years and years though. While search on this, I located this article from the GOA:
They're working to get the restrictions removed from ACoE oversight properties. Let's hope that this does pan out as there are ALOT of waterways and properties that are under ACoE's oversight.