"That big flat tail makes a helluva wollup!"

yeah, but it isn't flat when it hits the water - had a beaver watching my canoe once, and slapping his tail a number of times.  then he just held his tail poised above the water, ready to hit it,  and watched the canoe (on shore) for a minute - he had his tail cupped, like an upside down spoon.  I got a very good look at that.  not sure if they all do it that way or not, or always that way but this guy did.  normally the splat happens so fast that you can't really see how the tail is held.