Petroleum or other cargo is often transferred to or from tankers by mooring and cargo handling systems that lie in the sea and which are connected by pipelines that run along the sea bottom to the shore or a nearby installation. The system includes an anchored buoy to which a ship can be moored and one or more hoses that can extend to the ship. The system is normally constructed so that a ship that is moored to the buoy and connected to the cargo transfer hoses, can drift freely about the buoy under the influence of wind, waves, and current. The cargo-carrying hose or hoses can be connected between the ship and mooring foundation by way of a rotary fluid coupling or product distribution unit (PDU) which may be located underwater to isolate it from wave action. It is important that the rotatable portion of the PDU easily turn as the ship drifts about the buoy, to avoid wrapping the hose about the buoy, and yet this should be accomplished with minimal stress on the hose leading to the ship. U.S. Pat. No. 3,840,927 by Reid shows various underwater PDUs. U.S. Pat. No. 3,883,912 shows an underwater PDU with a hose arm coupling designed to maximize rotating torque with minimal stress on the hose, but it requires a pair of connections to the PDU which must be merged. A coupling of simple and light weight construction which aided in turning the rotatable portion of a PDU with minimal hose stresses, would facilitate the construction of reliable and low cost cargo transfer installations.