Where do Brits get the idea that you can't stand up in canals and if you fall into one you'll most likely drown?

Britons do not "get that idea" from anywhere; but most do have a healthy respect for water in which you can drown even if in fact you could stand up in it with your head above water.

The water in canals, mature rivers and lakes naturally carries fine sediment which makes it opaque, and that can make it look deep and dangerous. Which is a good thing in warning people to approach it with care.

Most fatalities in British waters are in rivers, lakes and of course the sea; and the vast majority are accidental.

As for that idiotic video, it was obviously staged.

Because if you see a low bridge you will lose your head