Swim diapers

I have a reusable swim diaper that my

Mom bought my little guy. He is 6.5 months. I was thinking of taking him to the pool in a few weeks but I’m confused a little bit on swim diapers. It says online it only keeps urine in. My baby is breastfed so his poop is still liquid. Wouldn’t the poop leak out if pee does? I obviously don’t want that to happen in a public pool but I’m a little confused at what the point of swim diapers are for baby’s under one since the poop would still be liquid.