Commentary for Avodah Zarah 145:13

According to Hizkiyah, if forbidden wine falls in first to a vat, then the entire mixture is prohibited. The fact that water falls in after will not change the fact that the entire mixture is prohibited. But if the forbidden wine and water are first mixed together, then this mixture will be permitted, assuming that the water nullifies the forbidden wine. If this entire mixture now falls into a vat of permitted wine, the entire vat remains pure.
R. Yohanan says that even in the first case, the mixture is permitted.