Montreal's definitely got a slightly different technique when it comes to bagel baking, but much of the difference of NY bagels is rumoured to the character of NY water, which amongst other things has comparatively heavy amounts of chlorine and fluoride, albeit it's originally from a decent deep mountain source. This is not entirely different from the composition of London water, nor as distinctive as something like Burton water
Equally, folk can similarly filter water to similar characteristics and have in the US, with varying results. It's fair to say that more than a few dismiss the water theory, pointing to banks of inconclusive blind tests and the fact that NY water isn't unique enough or sufficiently different to have that marked an effect in baking terms. Difficult to say for sure of course, but you would be sceptical that it makes a particular difference.