I'm afraid that's not totally true. There are some gas exchange through their skin, but the primary respiration is with the nostrils and lungs. If I recall correctly, there are one species of lungless frogs, but needless to say, they aren't exactlly the norm. It is very possible for them to drown when being submerged for a long peroid of time( one example is Bufo bufo, where females occasionally drown when several over-eager males all tries to mate at the same time, and the combined weight is too much for the female, so she drowns.