Kordox said: ...It's simply not possible for the water to stay pooled between her legs and her breasts in that position. This thing called gravity will make it run down. Koutaro is a good artist so I don't understand the reason for such an odd error.
My wager? Jelly. As in konnyaku or agar. You could make them semi-solid but still soft enough to suck from a straw... although pouring hot agar upon one's cleavage won't end well, that i'm sure.
He most likely used water as a neat way to censor this and that and besides the whole girl represents some consumables or rather a drink (at lest to me) who waits to drink her up. That straw and water between her breasts is most likely a a slight reference to paizuri. Demanding from artist obeying physics laws is like telling journalists to write grammatically which we know is impossible.