Reddit mentions of The Three-Year Swim Club: The Untold Story of Maui's Sugar Ditch Kids and Their Quest for Olympic Glory

My dad prefers biographies, historical books and non-fiction too. Last year for his birthday I got him a book about plantation kids on Hawaii who trained to become Olympic swimmers called The Three Year Swim Club and I got him a historical fiction book called Every Man Dies Alone that is about life in Berlin during WW2 and was written by a man who lived in German during the war. It was recommended as being a realistic fiction novel rather than a romanticized account.

Your FIL may also enjoy Stephen Ambrose's book about Eisenhower. Ambrose is an excelled historical writer.

I feel like "a body of water" == "a piece of flat land", and an olympic sized pool == a regulation, lined soccer field with properly sized nets. You can learn to swim anywhere (and most swimmers don't grow up or always train in swimming in olympic sized pools, which is more than double the length of a regulation high school/college pool, although they do have blocks and walls), just like you can learn soccer anywhere. And open water swimming is an olympic event, as long as you can find some way to mark the distance.

Semi-related: here's a book about some rando kids who learned to swim in a ditch, and ended up at the olympics. Full disclosure, I only read the back and the first chapter, and I wasn't drawn into the book, so I don't really know how their story shakes out.