Place almost any land tower on water with the Pontoon! Deploy the Pontoon on water, then place your land tower on top.
Official BTD5 Description
The Pontoon is a Special Agent that was released on May 17, 2012. It allows all land towers to be placed on water and costs $40 Monkey Money.
Pontoon Pro gives towers a range boost. You need 50 Pontoons to unlock it, which costs $2,000 Monkey Money.
You can't sell the pontoon until you sell the towers on top of it first.
This is one of the cheapest Special Agents (the others being Portable Lake and Monkey Farmer) costing $40 Monkey Money.
This is one of the few Special Agents that never attacks.
Pontoon is not sacrificed by the Temple of the Monkey God.
The Pontoon is basically useless on all tracks except Archipelago and Dockside.
Some say that the Portable Lake is better because the Portable Lake is helpful on many tracks while the Pontoon is helpful on two tracks but others would debate that Portable Lakes are only for the use of Monkey Bucaneers alone making them virtually useless.
It is possible to place a Pontoon on a Portable Lake but this is worthless and wastes $80 Monkey Money.
The art of the Pontoon has an inflatable tube underneath the Pontoon (probably because the Pontoon doesn't have enough buoyancy on its own to carry towers