פירוש על עבודה זרה 117:1

According to R. Yohanan, water belonging to the public cannot become prohibited, even if people worship it.

By deduction, if water owned by the public cannot become prohibited, water owned by an individual can. The problem is that in general things attached to the ground cannot become prohibited. Water is considered attached to the ground and thus should not become prohibited.
The Talmud answers that if a wave detached the water, and then people worshiped it, it can become prohibited.