Because fluid must flow past the wing, one uniform current would not be enough. The seastead would drift with it, and the water would appear still. In order to sail, we need a varying current, so that we can use the differences to generate motion in other directions. Fortunately, it is not uncommon for the currents at the very surface of the ocean to be different than deeper currents. The thermocline, a region of rapid temperature change, is usually 10-200m down, and it seperates the surface "mixed layer" from deeper waters. Currents are often different above and below the thermocline.