My wife had an interesting theory. If you suddenly pop up the North American plate, you might get a cavity or two below that water could drain into. The other theory is that the crutal motion that produced the tsunami also created large turbulence with crosscurrents, not unlike tornado formation, hence a funnel. I like the cavity theory because it’s scarier, but the turbulence theory is probably correct.It's called an eddy. It's caused when the main current goes one way but something causes a portion to peel off and go the other way. In this case, the something was structures cropped from the photo.