Place almost any land tower on water with the Pontoon! Deploy the Pontoon on water, then place your land tower on top.
~ BTD5 and BTD6 description
The Pontoon is an object appearing in Bloons TD 5 and Bloons TD 6. It is depicted as a few wooden logs floating on an inner tube. When placed in the water, land towers can be placed atop the Pontoon. Although it has a pop count, the Pontoon has no attacks. The Portable Lake is the opposite of the Pontoon, allowing water towers to be placed on land.
In Bloons TD 5, the Pontoon is a Special Agent that was released on May 17, 2012. It costs 40, making it one of the cheapest Special Agents, alongside the Monkey Farmer and Portable Lake.
In Bloons TD 6, the Pontoon is available as a Power and costs 50. In this game, selling a Pontoon or Portable Lake will also sell the towers on top of them.
Some say that the Portable Lake is better because the Portable Lake is helpful on many tracks with more land while the Pontoon is only helpful on tracks filled with water like Archipelago or Spice Islands, but others would debate that Portable Lakes are only for the use of Monkey Buccaneers and/or Monkey Sub, making them virtually useless.
It is possible to place a Pontoon on a Portable Lake but this is worthless and wastes 80/40 if you spent it on Monkey Money.