Please ask your baby and toddler swim school and/or leisure centre to confirm their nappy rule (sometimes referred to as the Double Nappy system).  It is becoming mandatory in many swimming pool environments that all babies/young toddlers wear a disposable swim nappy underneath their reusable swim nappy.  This means the disposable (eg Huggies Little Swimmers) is worn next to baby's skin and the reusable swim nappy is worn on top as the outer layer.   Huggies have launched (June 2008) a new extra small size for babies under 16lbs.

Avoiding leaks are important as the swimming pool will have to close down should any soilage leak into the pool due to the risk of infection.