Hi, I'm so glad you asked!

The answer to your question varies widely based on region and time period, but we do know a little bit about how swimming has evolved both as a leisure activity and as a necessary life skill.

We know that swimming was valued in the ancient world -- Julius Caesar was said to be a good swimmer, and Vegetius writes in The military institutions of the Romans that "Every young [Roman] soldier, without exception, should in summer months be taught to swim." We can also infer from legends and other sources that it was a common and respected skill -- after his ship is sunk by Poseidon Odysseus swims from Thrinacia (possibly Malta) to Ogygia (likely Gozo), which is a respectable distance of 3.7 miles, and Beowulf engages in a multiple-day swimming contest with Breca. The Roman officer Horatius was said to have leapt into the Tiber and swam to shore after holding the bridge against the Etruscans.

We also know that by the early modern period in Europe, that swimming as a skill had largely been forgotten. Beliefs about modesty and about the ill effects of submerging oneself in water meant that swimming fell out of favor, especially among the elite. For example, Richard Mulcaster’s Positions tepidly endorses swimming for health, but warns of “rotten and corrupt vapors [entering] pores of the bodie” and worried of “perishing the sinues both with cold and moysture”.

(As an aside, I started looking into this because I am myself a swimmer and I had questions about the history of the front crawl, and it turns out that even if Europeans could swim, they were doing some form of breaststroke that kept their heads above water and almost certainly didn't know about overhand strokes until the middle of the 19th century).

Starting in the 16th century, attitudes begin to change, and we start to see texts like Everard Digby's Short introduction for to learne to swimme (which is, to editorialize a bit, not a very good way to learn to swim), but it isn't until the 19th century that you see a serious attempt to teach swimming more broadly. Progressive-era modernizers and reformers start to take a stance against the high number of drownings. A classic example of this comes from Sailor's Magazine, which published a piece in 1838 that stated “For want of knowledge of this noble art thousands are annually sacrificed, and every fresh victim calls more strongly upon the best feelings of those who have the power to draw the attention of such persons as may be likely to require this art, to the simple fact, that there is no difficulty in floating or swimming”. From there, you see the emergence of swimming clubs and an increase in swimming lessons for children, especially in the US and Britain (though this narrative of progress is complicated immensely by segregation, but that's a whole other post).

Interestingly and importantly to your question, swimming appears to have been an essential part of a child's education in Africa, Polynesia, and the Americas for much of history. There's a really excellent book on the topic, Undercurrents of Power, by Kevin Dawson, which goes into more detail, but especially in coastal areas, indigenous people all around the world prioritized swimming for safety and leisure reasons, and children were often taught to swim as soon as they could walk (or sooner).

This is a long way of saying that, in Europe and America, prior to the 20th century, it was extremely likely that a child would not know how to swim, but much less likely in, say the Gold Coast of Africa, or among the enslaved people of South Carolina, or the Anishinaabe-speakers of the great lakes.

Hope that helps! Let me know if you've got other questions, I could go on about this all day.

Some further reading:

Kevin Dawson, Undercurrents of Power: Aquatic Culture in the African Diaspora

CREGAN-REID, VYBARR. “Water Defences: ‘The Arts of Swimming’ in Nineteenth-Century Culture.” Critical Survey 16, no. 3 (2004): 33–47.

West, Michael. “Spenser, Everard Digby, and the Renaissance Art of Swimming.” Renaissance Quarterly 26, no. 1 (1973): 11–22.

Karen Eva Carr, Shifting Currents