Just to calrify- are you talking about everyone worldwide, or just the entirity of a specific population?

If you are thinking about places like Australia, many schools have had mandatory swimming lessons for all children from a very young age, for about two decades now.

However, I should itterate that it is not assumed that everyone can swim. In fact, drowning in dams, pools and beaches is unfortunately common, specially with the children of immigrants who may not have been able to access swimming classes early in life, and those who come from places with no swimming avaliable at all, such as desert and rural areas.

A better question to ask would be about "swimming culture" in anglicised countries, to which there is plenty of information if you google it. I know the Australian government website has quite a few pages on it, with references to our beach cultures, swimming in the olympics increasing pool visitor rates, and civil rights affecting the ability for aboriginal children to swim.

http://australia.gov.au/about-australia/australian-story/australias-swimming-success-pools

I'm not sure that your premise is true, i.e. I'm not sure it is commonly accepted.