פירוש על עבודה זרה 117:4

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.

Earlier, on daf 46 there was a discussion about boulders that had become detached from a mountain on their own force, without human intervention. Can such boulders become prohibited if worshiped? Water that becomes detached by a wave is similar—a natural feature that becomes detached from its source on its own. So if R. Yohanan holds that it is prohibited, he must be the one who held that boulders detached from the ground are prohibited.
The Talmud again refines the case—the water worshiped here was not detached simply by a wave. Rather, a non-Jew struck the water, then worshiped the detached water. Can a Jew now use such water? R. Yohanan would say yes, but this does not mean that he is the one that holds that the boulders are permitted.

The three practices that R. Hiyya sees are what I would call “marginal.” The women are not becoming pregnant from non-Jews. They are having children with non-Jews who seem to have converted, but not done so according to rabbinic law. They know that conversion entails circumcision, but they do not know about immersion.
They are not drinking wine made by non-Jews. They seem to know that this is prohibited. But they do not know that it is also prohibited to drink wine poured by non-Jews.
They are not eating non-kosher food. They are eating lupines, a type of bean, cooked by non-Jews.
All of these are what I would call infractions of rabbinic law, practices that many Jews may simply not have known about. They are also all intended to keep Jews from fraternizing with non-Jews.

Yohanan adopts a far more strict approach, ruling strictly in each case. The Talmud will now analyze each issue separately.

Yohanan holds that until a convert circumcises and immerses he is considered a non-Jew. So these women were pregnant from non-Jews. And R. Yohanan also holds that if a non-Jewish man impregnates a Jewish woman, the offspring is a mamzer. So these children are mamzerim. Note that this is not the accepted halakhah.

As we learned earlier, non-Jews should not even mix wine for Jews because it is just better to stay away from this type of situation. Here we see that R. Yohanan goes a step further and deems such wine to be yayin nesekh.

According to R. Yohanan the lupines cooked by Gentiles are prohibited only because the people of Gavla are not “students of Torah.” The implication is that they are lax in their observance and in consequence, the halakhah must be extra stringent with them. But, as we shall see, lupines should have been prohibited in any case.

Lupines cannot be eaten raw. Therefore, the prohibition of food cooked by Gentiles should, according to a statement made by Rav, apply to them. This was an issue more fully discussed way back on 38a.

There is another version of what types of foods are prohibited if cooked by Gentiles—foods that kings use to accompany bread. Simple lupines, a type of bean, are not eaten by kings. Therefore, Jews may eat lupines cooked by non-Jews. However, R. Yohanan prohibited the Jews of Gavla because they were not students of Torah. He thought that if they heard of this leniency they might come to transgress other laws.

In principle, a non-Jew can carry grapes to the winepress because they are not yet wine. But R. Kahana says we should not let them do so just to stay away from any chance of them coming to touch the wine.

The baraita clearly allows grapes carried by non-Jews to the winepress. Even if some liquid comes out of the grapes, the grapes do not become yayin nesekh.