Swim diapers are specifically designed to contain poop and prevent it from entering the pool water, which is essential for public hygiene. However, keep in mind that they do not absorb urine like regular diapers. If you are certain your baby has already had a bowel movement for the day and is unlikely to poop again, you can sometimes skip the swim diaper and just use a swimsuit, but for general pool play, swim diapers are the standard recommendation.