These are barrage balloons. A barrage balloon was a lighter-than-air balloon tethered to the ground or a ship by a long cable. The hope was that any low-flying aircraft would become entangled with the cable. This would cause damage to the aircraft, possibly causing it to crash. The balloons were intended to be used as part of a multi-part defense system. Low-flying aircraft would be deterred by the balloons, and forced to higher altitudes, where they became easier targets for AA guns. They were typically placed around around targets of high strategic value. As you've observed here, they were also fitted to ships for inshore amphibious operations, like D-Day, or to protect coastal convoys.

The typical British barrage balloon was constructed of a rubberised cotton fabric, coated in several layers of a silver paint-like dope to make it gas tight. It was divided into two sections, one filled with hydrogen gas to provide lift, and the other by air. It was attached by a long steel cable to a base station. These were typically Fordson 'Sussex' six-wheel trucks, fitted with a high-powered winch. For water-borne use, commandeered barges were used for protecting coastal anchorages. Convoys were protected by hired merchant ships fitted with the balloon equipment. In the Humber estuary, the paddle steamer HMS Killingholme was used as a balloon station. For the D-Day landings, many landing ships were fitted with the hoisting equipment.

The cables for British balloons in WW2 were not simply passive devices. They were fitted with explosive charges 150 ft above the ground, and the same distance below the balloon, with two parachutes fitted between the links. The shock of an aircraft impacting the cable would cause the charges to detonate, severing the cable. The parachutes would then open, greatly increasing the drag on the aircraft towing them, and hopefully causing it to crash. However, if the charges were accidentally triggered, then the wandering balloon and cable could cause havoc on the ground. All told, they were ineffective at downing enemy aircraft, though their effectiveness at forcing aircraft to higher levels is harder to measure.

Screwed the image linking up somehow, my bad :(