pon·toon 
(pŏn-tn)
(pŏn-tn)n.
Any of various floating structures, especially:
a. A flatbottom boat used to support a bridge.
b. A floating structure that serves as a dock.
c. Either of a pair of floats supporting a boat or seaplane.
[French ponton, from Old French, from Latin pontō, pontōn-, floating bridge, from pōns, pont-, bridge; see pent- in the Appendix of Indo-European roots.]
pontoon