I never gave thought to it before, but it makes sense they would be able to float for a while, being so light for their volume. Even the Fish Owls and Fishing Owls try to avoid getting wet though, so it is not a common occurrence.
Here's a short Audubon article about an adult being forced to swim after an unsuccessful nest takeover attempt. It has a very short video clip of an adult GHO doing the breast stroke.