I can't remember exactly how old we were (I may have been 7ish and he may have been 4ish. I remember it was before my dad lost access to us and I was 8 when that happened).
Anyways, we were at our paternal grandparents swimming in their inground pool. Our grandpa (grandma was at work) left us unsupervised and was doing work in the garage and RV, packing for our trip that weekend.
My brother decided to jump off the diving board and he couldn't swim. I remember hearing a splash and turning around. He couldn't stay afloat and was going under, I remember panicing and screaming for my grandpas help but he didn't come, so I swam to him as quickly as I could. I made him climb on my back and swam us to the edge where we climbed out.
It was one of the scariest memory of my childhood.