Avodah Zarah 59A

דאמר רבי יוחנן משום רבי שמעון בן יהוצדק: מים של רבים אין נאסרין. הא דיחיד נאסרין.

as R' Yoḥanan says in the name of R' Shimon ben Yehotzadak: Water that belongs to the public is not rendered prohibited. The Talmud infers that since water that belongs to the public is permitted, therefore, in a case where non-Jews bow to water that is owned by an individual it is rendered prohibited.

ותיפוק ליה, דהא מחוברין נינהו! לא צריכא, דתלשינהו גלא.

The Talmud challenges: But R' Yoḥanan could derive that even water owned by an individual is permitted, as the water is connected to the ground, and worshipping an object that is connected to the ground does not render it prohibited. The Talmud explains: No, it is necessary to derive this halakha from the fact that the water belongs to the public in a case where a wave raised the water and detached it from the ground. In this case worshipping water owned by an individual would render it prohibited.

סוף סוף, אבני הר שנדלדלו נינהו, תסתיים דרבי יוחנן דאמר אסורות?

The Talmud challenges: Nevertheless, the water ultimately falls into the category of objects that were detached without human involvement, such as boulders of a mountain that dislodged on their own. The Talmud (46a) cites a dispute between R' Yoḥanan and the sons of R' Ḥiyya with regard to boulders that dislodged without human involvement and were then worshipped, and does not conclude who deems the boulders permitted and who deems them prohibited. May it be concluded from R' Yoḥanan’s statement that it is R' Yoḥanan who says that the boulders are prohibited?

לא צריכא, דטפחינהו בידיה.

The Talmud replies: No, even if R' Yoḥanan deems the boulders permitted, his ruling with regard to the water is necessary in a case where one struck the water with his hand and thereby detached it. Since it was detached due to human involvement, if the water was owned by an individual it is prohibited.