The average hot air balloon has a volume of 80,000 cubic feet.  If submerged in water and filled with oil, a balloon of the same size would contain about 20,000 barrels of oil.  Underwater balloons would provide a fast and easy way to capture oil from underwater leaks while a longer term solution (capping or relief well drilling) is worked on.  The balloons work with the natural buoyancy of the oil, rather than against it (as BP's first containment dome attempted to do).  In a sense, the idea is not new, it's just a twist on something that's been done successfully before.  The Bathyscaphe Triest was basically an underwater zeppelin.  Flotation of the Deepwater Horizon itself was provided by 4 pontoons filled with 700,000 gallons of diesel oil.  A pontoon is little more than a fabric or metal balloon, used to provide buoyancy in water.  The biggest issue to be overcome with this approach, is the presence of dissolved natural gas and/or methane hydrates in the balloon.  As the balloon rises and the pressure within it falls, these gasses will bubble out and expand, potentially forcing the oil out of the bottom of the balloon.  Gas relief valves (which allow gas to escape, but not liquid). built into the top of the balloon should be a relatively simple solution to this problem.