I can't see how Sauron could "summon" them.
They would try to return to Mordor, as they did after the event on the Ford of Bruinen
But I wonder how water would influence their powers. We know they hated water, and Tolkien was aware that the way they managed to cross the dangerous ford at Tharbad is more or less a plot hole.
So what would they do in so much water...?
I don't really know.
