An instrument-carrying vehicle capable of being towed at depths of a kilometer at speeds of 5-6 knots is described. The vehicle system can be operated from medium-sized vessels without special handling equipment, cumbersome and expensive cable fairings generally considered necessary for deep and/or fast tows are unnecessary. A stream-driven propellor coupled to a hydraulic pump provides power for actuation of control surfaces. Control is by electrical command from the surface. Instruments including conventional STDs, velocimeters and a current meter have been fitted to the vehicle system. A smaller body is towed below the vehicle to provide a second platform for gradient measurements at fixed vertical separations. Depth is surface controllable over plus or minus 30 meters of nominal without the necessity for hauling-in or paying-out wire. Stability at any depth has been shown to be excellent. It has been possible to follow isodensity (isopycnic) surfaces continuously and track them in depth.