Swim diapers are only meant to catch feces, not urine. So with any type of swim diaper, you'll probably want to wait to put it on your child until you arrive at the pool, to avoid wet clothes and car seat.
If you're using a pocket diaper without an insert as a swim diaper, you will want to designate one diaper as your swim diaper, not rotate it through again with your other diapers, as the chlorine in public pools will affect the waterproofness. Since you'll use it without an insert, you also may want to choose a diaper your child has almost grown out of and designate that as your swim diaper. You want to make sure there's a good fit around the legs, so you're not responsible for that loudspeaker announcement, "Please clear the pool due to fecal contamination." :-)
