SoSecure swim diaper (fecal containment system) is the best reusable.
Swimsters and Swimmates are good disposables. Roughly equal in quality but not nearly so effective as the resizable swim diapers.
As previously mentioned in another thread, fit is extremely critical.
How do you know they fit right? A tight fit at the legs is a must. Of course, a reasonably good fit at the waist also is necessary.
Put on a dry swim diaper. Pour a small quantity of water in the waist. If it does not leak out the legs it probably will contain fecal matter "long enough."
If you really are worried, give them a "field test" while in the bath or shower. If you don't see any "floaters" they work.
--John