I think the rule for water blocks is, if it’s the top surface of a water block, it renders with the transparent shader. If it’s the side, the assumption is that the water is flowing, which gets rendered with the flowing water texture.
Unfortunately water up against the side of a glass, which shouldn’t be flowing, it shown with a flowing texture.
I am not sure it can be rendered with the top surface shader due to performance concerns, but I do think that maybe a transparent nonreflective shader would better represent the physical reality of what’s going on.
The specific concern is that underwater tunnels look great if you look up, you can see through the water and it’s awesome. But from the side it’s just blue.