An apparatus for maintaining at a predetermined depth a tow cable, such as that of a seismic streamer, being towed by a vessel, the apparatus having one or more depth controller devices attached to the cable at spaced intervals therealong and a remote command transmitter on the tow vessel for transmitting commands to the depth controller devices, the depth controller devices having a mechanism responsive to deviations between the commanded depth of the cable and its actual depth for moving diving planes of the controller devices to cause the controller devices to move to the commanded depth as the cable is towed along by the vessel.