A deflector device (22) for use with a tow line between a seismic survey vessel and a tow, in particular a seismic streamer or streamer array, in the water behind the vessel comprises a vertically oriented wing-shaped body (28) shaped to produce in use a sideways force which urges the tow line laterally with respect to the direction of movement of the towing vessel. The wing-shaped body (28) includes one or more buoyancy elements, and a rearwardly extending boom (32). A pivotable control surface (54) extends sideways from the boom (32), and is shaped to produce in use a force having a substantial vertical component. The angle of the control surface is remotely controllable, in order to control the depth of the deflector device.