Fiber-optic cable is dispensed underwater into a trench from a vehicle moving along the bed of the body of water. The cable is wound on a stationary spool and dispensed over the end of the spool by a rotary arm driven by a motor controlled to maintain a desired tension in the cable. The cable has a fiber optic core, metal wires wound around the core, and a tensioned laminate of copper tape adhered to plastic tape wound around the wires.