The simple answer to your question is yes, if you take a lengthy shower you would become a Sheni l'Tumah d'Rabanan (Rambam, Hilchos She'ar Avos ha'Tum'os 9:1). And, by the way, if you go swimming in a swimming pool and dive in, the same thing would happen. But it is not very relevant to us because it only applies to people eating Terumah or Kodshim. Furthermore, we are all presumed to have a much more Chamur Tum'ah, Tum'as Mes.

This is one of the 18 Gezeiros made in the attic of Chananyah ben Chizkiyah ben Guryon. It is a Davar sheb'Minyan -- a Halachah that was enacted by the Beis Din ha'Gadol -- and therefore remains in effect until another Beis Din even greater than it overturns it, which has not happened yet. So even though today most of the laws of Tum'ah and Taharah are not applicable, and the Kohanim do not eat Terumah, nonetheless it remains in effect.

The question is whether the water coming out of our plumbing today is Mayim She'uvim or not. The general assumption is that it is, because probably somewhere along the line the water went through a Beis Kibul in the piping (for example, the water might go through a water meter that has a Beis Kibul). And even though the plumbing is Mechubar l'Karka, since the pipes were made before they were installed they retain the status of a Kli. So all of the water coming out of the shower would be Mayim She'uvim from one Kli. This is why we cannot fill up a Mikvah with water from the tap.

The only way this Gezeirah impacts the Halachah today is in the area of a woman going to the Mikvah. In Hilchos Mikva'os (YD 201:75), the Rema rules that a woman should not wash off after going to the Mikvah. The Vilna Gaon, commenting there, argues with the Rema and says that the Gezeirah was made only for Tevilah for Terumah, whereas a woman's Tevilah for the purpose of removing the Isur Nidah is only for Chulin (and therefore she does not need Kavanah that she is immersing, as does a person immersing for Terumah).

Kol Tuv,

Yonasan Sigler

This is not a Psak Halachah