I've seen those before, not on the water but on construction sites, most of them been up that way They call them wind devils, kind of like mini tornados. Weather guy told me they occur a lot on in that area because of the warm west winds coming across the Piedmont then the wind hits the James which is still cold this time of the year. I worked a job near Ft Eustis back in the 80's one would come through every couple of days, one was strong enough to blow my wheel barrow over and send cylinder molds flying through the trees. Those are some neat pics of one!