Can I use a cloth diaper cover or a pocket diaper without an insert for a swim diaper?

Yes, either of those would work fine for a swim diaper. However, once the diaper has been used as a swim diaper, we don't suggest putting it back into your rotation of cloth diapers. The chemicals in pools can be hard on elastic and PUL so that it may no longer be waterproof and it could get on your other diapers, causing them to wear prematurely.