Today, people associate pontoons with hollow platforms designed to float on water, or a type of boat made out of pontoon-like structures, but the word originally meant "bridge". Over time, the definition got increasingly specific to the point where it only referred to military bridges made out of those platform things, and then the term got associated with the components, giving us the modern form. The word was borrowed in a 1590 war philosophy book from Old French ponton, which in turn came from Latin pons, also meaning "bridge" (we've seen this element before in pontifex, "bridge-maker"). That's reconstructed to the Proto-Indo-European root pent, or "path". The word pontoon peaked in usage during the Civil War and has been relatively stagnant in recent years.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.