Ferrara's dad is correct according to a former Neapolitan I know- it is a thundermug (chamberpot) in Neapolitan.
In Italian, vasca da bagno is a bathtub; also just bagno is a bath or a tub. Vasca by itself is a basin. But each area has its own dialect. Neapolitan is almost a separate language and there are other dialects down towards the boot heel.
He looked at the words by Allen and Merrell and said some weren't the Neapolitan he knew; as suggested above, some may catch the sound but not the spelling.
Lou Monte and Dean Martin had a lot of fun with this song in public appearances, adding more dialect with double meanings.

Another word that stops North Americans cold is douche, often used by the Dutch to mean to take a shower. It is, or was, taught in their English classes. Perhaps it does refer to a shower in some areas of the UK.