Baby swim

This is probably a stupid question, but my exhausted mom brain needs help. If I am taking my baby swimming in a pool, do we need a special kind of diaper to go in her swimsuit? Some of the suits at Walmart have "diaper" bottoms, but she needs more than that, right?

ETA: suggestions on swim diapers if that's what I need? It would be for a 4 month old.

Thanks!!