pontoon

Short definition from Wiktionary

Noun

1. 1
   A flat-bottomed boat or other floating structure used as a buoyant support for a temporary bridge, dock or landing stage.
2. 2
   A bridge with floating supports.
3. 3
   A box used to raise a sunken vessel.
4. 4
   A float of a seaplane.
5. 5
   A lighter or barge used for loading or unloading ships.
6. 6
   A card game in which the object is to obtain cards whose value adds up to, or nearly to, 21 but not exceed it.