As waterways were the only efficient means of transportation before the age of trains, you will find most larger cities in Europe near a river or lake or at lea

As waterways were the only efficient means of transportation before the age of trains, you will find most larger cities in Europe near a river or lake or at least small waterway. And beyond that, there is a plenty of lakes and small ponds. So I would guess rather 90% of the population lives and lived close to a place where they could swim.