If a parish practices full immersion or the kind of pouring where the candidate stands in a shallow basin and a gallon or so of water is poured over the head it would be ridiculous come to Mass wearing one’s best clothing only to have it ruined by a full dunking! This is true whether the candidate is an infant, a child, or an adult. Wet clothing, particularly women’s, is likely to be quite immodest when wet, not to mention cold. This is why such parishes have the babies baptized naked or in just a diaper. And such parishes usually have older candidates wear flip flops and a bathing suit (or similar) under a brown robe. (They are baptized with the robe on so no one SEE’s the attire underneath.) The newly baptized would then change into a dry white robe (with appropriate attire underneath) or into other appropriate attire that reflects the new status of the baptized.