What does water do to wine?

Water dilutes wine by watering it down; even an expensive French wine becomes something probably not worth trying to swallow. The context uses this to picture corruption, showing that adding water spoils the wine's value and quality. It is no longer desirable or worthwhile as wine.

Les asks what water does to wine. He says he is not a buff on wine, but he can imagine that if someone has an expensive French wine and waters it down with water, what is left is something probably not even worth trying to swallow. In the surrounding context he ties this to corruption—somebody corrupting them—and says that is exactly what he is talking about back in Galatians. The illustration shows that adding water spoils good wine and makes it undesirable.

Isaiah 1:22

Thy silver is become dross, thy wine mixed with water:

What was the allusion behind the water and wine image?

The water and wine image alludes to corrupting or diluting something pure, as when wine is watered down and becomes worthless. It represents false teachers corrupting believers, which Paul directly addresses in Galatians, strongly contrasting with his own uncorrupted spiritual commission.