I've noticed that quite a few people (younger that I am!) have little grasp of history, or engineering for that matter, which would explain the 'tanks' confusion.
Armoured Fighting Vehicles were originally code-named 'tanks' whilst being built in numbers for the first time, in an attempt to hide their purpose from spies during WWI. So I understand.
Civilian English people aren't very fond of acronyms, and the name stuck.
There once were 'swimming tanks' that floated.

Sorry, it just triggered an old memory.