I think the kids would do well to research the actual laws in place, rather than relying on a police officer who may not know the actual regulation. Or, at a minimum, speak with the local game warden. The landowner should do the same.
I live in a state where the water is not owned, but the land underneath it is.
I previously lived in a state where some waterways were publicly accessible below the average high water mark. Those streams were designated based on their ability to be navigated. Not all streams made the cut.
I believe Montana has a fairly generous stream access law.
In the case of this video, it seems to me that they probably knew that they were going to have a confrontation ahead of time.
Edit:
A quick google search suggests that New Jersey (assuming the above post location is correct) also regulates non-tidal waterways in that the water itself is accessible, if floating. Private landowners own the surface beneath the water. Funny, that the landowner seemingly had not bothered to research the actual law in order to articulate why they were trespassing.