pontoon1

noun

• 1a flat-bottomed boat or hollow metal cylinder used with others to support a temporary bridge or floating landing stage: [as modifier]:a pontoon bridge
• a bridge or landing stage supported by pontoons.
• each of a pair of floats fitted to an aircraft to enable it to land on water.

• 2a large flat-bottomed barge or lighter equipped with cranes and tackle for careening ships and salvage work.