Thanks Luis for finding and posting these obscure and unique events.
This a good one. A one time shot.
At first I smiled and thought no way would that work.
Turns out it did really.
Kind of.
From Tank-AFV.com
"The world's first amphibious APC
Shortly before the end of the war the first Mk.IX tested the ability to install floats for wading. As such, it was emptied, and reinforced on the flanks and fore part of the hull. The landing doors were sealed and gaskets used for pumping air. Movement in water was provided by the rotation of the tracks, but special blades attached to the links were added for extra grip. In addition, a roof compartment housed part of the gear-cutting equipment, and tailpipes get through it. The amphibious Mk.IX was unofficially called "The Duck" and began a test series in November 11, 1918, conducted at Dolly Hill. These were considered successful, although the tank was very slow and had low water buoyancy. In addition, this configuration precluded placement of access doors and the installation of a powerful weaponry. In November 1918 the tests were terminated. The only amphibious Mk.IX was subsequently scrapped, but the data obtained in the tests helped later in the construction of more advanced amphibious tanks by Vickers. So far, only one Mk.IX noted IC 15 survived, now exhibited at the Bovington Tank Museum."
A minute and a half video:
British WW1 Tank Mark IX Armoured Personal Carrier (1918)
tank-afv.com