English Russia reports that during the 1950s, the Soviet Union developed floating tanks. Engineers attached pontoons to T-54 tanks and drove them into the water:
PST-U consisted of five pontoons that were filled with plastic foam. Total weight of the device was 10 tonnes. Buoyancy reserve (with T-54 tank) was 40%. Maximum speed of tank with the floating device was 19 km/h ashore and 12 km/h afloat. The floating device was equipped with its own fuel tanks with a capacity of 500 litres; equal to coverage of 60-80 kilometers distance without any tank’s fuel consumption.
At the link, you can find more pictures and diagrams.
Link | Photo: English Russia