Nowhere to float your boat? Smart monkeys know they can place a Portable Lake on land, allowing any water unit to be deployed within its waters.
~ Official BTD5 Description
The Portable Lake (also known as the Portable Pool) is a Special Agent that was released on May 17, 2012 in an update to Bloons Tower Defense 5, and also appears in Bloons TD 6 as a Power. It allows water towers to be placed on land. It costs 40 to purchase one. Fifty Portable Lakes are required to be placed in order to unlock the pro version, the Sea Monster. In BTD6, it costs 50 (40 with "Cheaper Lakes" MK).
A maximum of five Monkey Buccaneers can be placed in one Portable Lake; in order to get this to work the first two to three Monkey Buccaneers need to be angled to the left or right.
A maximum of seven Monkey Subs can be placed in one Portable Lake; in order to get this to work the first one needs to be in the middle bottom, the second two at the bottom sides, the third two above the second two and the last one in the middle top, the seventh can be placed in the exact middle.
If the quality of the game is changed to low, a maximum of eleven Monkey Subs can be placed in one Portable Lake; in order to get this to work the first one needs to be in the middle bottom, the second four at the right side, the third four at the left side and the last two in the middle. For help with placing the subs look at the picture.
This is because the only Special Agents that can be put in water are the Pontoon and the Tribal Turtle, however, the Tribal Turtle can be placed on land also, and the Pontoon isn't allowed in Special Agents Only.
With the Support Chinook's Special Ability of moving towers, the player can place a tower in the Portable Lake, move the lake with the Special ability, and have the towers in the lake remain where they are.
In BTD6, selling a Portable Lake or Pontoon will also sell the towers on top of them.