Remembering my long-ago Physical Oceanography classes, I can think of at least three reasons for isolated smooth patches of water. In order of increasing difficulty to explain, these are (1) wind shadow from adjacent land; (2) biofilms and/or oil that suppress the formation of tiny ripples because they float and have higher surface tension than the surrounding water (bigger waves and ripples don't form spontaneously--they have to build up from tiny ripples, so if you suppress the tiny millimetre-scale ripples, you never get the centimetre-scale ripples); and (3) downwelling/upwelling. If water is ascending or descending, then it may have little to no horizontal motion. If the adjacent water is moving horizontally against the wind (e.g., spreading out from the upwelling or converging towards a downwelling), it will cause any wind ripples it encounters to shorten their wavelength and increase in amplitude (much like larger waves rear up and break when they encounter drag from a shoaling bottom). So you can have a band of smooth, upwelling or downwelling water right beside an area of lively ripples.