Danward, yes intuitively it wouldn't seem right for velocity to be greatest near the center. I'm a hydrogeology major and am helping my profesor doing research in fluvial systems (river, stream and channel dynamics). As a precursor, we had to mesure stream velocities at differenct depths with a tool resembling a wind vane. If you would like, I can refer you to some geomorphology books that deal specifically with channel dynamics. I apologize, i should have elaborated a bit. It's not the fact that it's a fluid density boundary, but the fact that turbulence results from this density difference. Water flows fastest where there is the least amount of turbulence. Wind causes a slightly different velocity and direction of water on the surface and this creates turbulance. This turbulence acts as a force against the current. If there wasn't any air acting on the surface of the water, what you said would be absolutely correct.

Jake, i guess i'm not sure what you mean by boat in a bucket? If your boat is in the ocean with now wind. There is no force acting on it. You can point the boat in any directin and the boat will not move. But in a channel, the only way to minimize the force of the water is to sail straight up stream. You can easily hang onto your boat when it's pointed up stream, but if you hold your boat side ways to the current you will be dragged downstream. Boards down provides more wetted surface for the current to push on if your angle to the current is anywhere but straight up. I could draw this out and post an attachment if anyone is curious.