One of the earliest decisions made by the Supremes was that Tribal fishing rights included access across private property to fish, including erection of temporary structures to catch and process fish. If memory serves, this decision was handed down in the late 1800s or early 1900s.

I don't believe that they can make permanent alterations, but could probably do a temporary launch annually, if that was necessary to access U&A. The QIN may be your friend in this argument.