Sure, but they probably didn't water the whole garden with them in a situation where we would turn the hose on. A watering can is good for watering a single plant or small bed, when that's all that's needed. It's not an efficient way to do a mass watering.In somewhere like Medieval England they definitely used 'watering cans' of various sorts (except they were not cans but pots - being made from ceramics).
In the illustration at the second link, they're using the watering can to water a seedling. Which is, probably, what the watering cans were mainly for (really no different today). Plants do need to be watered when they've just been put in the ground, and it makes sense to water them individually. For regular irrigation of an established garden, a quicker method would suit better.Here is a link to a late Medieval/Tudor example in the Museum of London.
Here is another link to an article on watering in medieval and early modern times, with some pictures of the implements they used.
For added nerdiness get yourself a copy of Thomas Hill's The Garden Labyrinth (written in 1577). He wrote, on the subject of watering gardens:
The common watering potte for the Garden beddes with us, hath a narrow necke, bigge belly, somewhat large bottome, and full of little holes, with a proper hole formed on the head, to take in the water, whiche filled full, and the thombe layde on the hole to keepe in the aire, may on such wise be carried in handsome manner...