the depth of the streamer 11 as desired the winch control system 23 depressor control system 24 and altitude keeper control systems 25 are controlled by the main control system 20 to safely lower tow and raise the streamer 11 and associated devices to perform a survey fig2 shows a depth controller suitable for use in a system according to an embodiment in the form of a depressor the depressor 30 is provided with control surfaces 31 to enable the position of the depressor in the water to be altered the depressor has a tow point 32 from which it can be attached to the towing vessel for example via tow line 13 of fig1 the depressor is also provided with instrumentation as described previously and a control system not shown fig3 shows an altitude keeper suitable for use in a system according to an embodiment which takes the form of a winged towfish the towfish 40 comprises two approximately symmetrically shaped parts which are provided with control surfaces 41 the two parts of the towfish 40 are separated by a gap 42 and are attached to each other only at attachment point 44 the approximate position of which is indicated when the towfish is in use the streamer 43 is accommodated in gap 42 and is attached to the towfish at attachment point 44 the towfish is also provided with appropriate instrumentation as referred to above and a control system not shown it.