If the willows have leaves out of the water, they will likely live and grow with the branches/trunk under the water developing new roots.
My only complaint with willow oaks are that branches will start new trees in the water, especially at the edges. Like cattails, they can't simply be cut down to kill them and they can become very invasive. They sure are pretty though, make awesome habitat, and are nice and cool to sit under on a hot summer day!
