During underwater foundation preparation, it is sometimes necessary to provide a perfectly prepared and clean surface.  This may require bulk removal of sand, gravel, cobbles, or the like.  This can be efficiently achieved using an underwater vacuum cleaner, for example an air lift, or water lift as shown (underwater photo courtesy of Chris Streater). 

The water lift is generally more efficient in shallower water and permits use of lighter equipment, as there is less chance of the water lift becoming buoyant.  The water lift operates upon the venturi effect.  In the same way that air-flow through a carburettor facilitates the transfer of fuel, water-flow through the lift facilitates transfer of sand, gravel and even cobble-sized stones.