As other comments have mentioned, a glass of water tells you nothing about the conditions outside the plane, which are the ones that matter.
But also, it is sometimes useless at telling you about the conditions inside the plane too. If you're rolling and yawing at the same time (known as a coordinated turn), the water in the glass will stay level (relative to the bottom of the glass), and the glass will not slide along its surface, even in a 20 degree bank.
But if you’re in straight and level unaccelerated flight, and the wind outside has no vertical component, the glass of water would allow you to deduce your angle of attack (namely, the angle of the water plus the angle of incidence, if I’m not mistaken).
Of course, that doesn’t help you at all during dynamic or unusual situations.
But also, it is sometimes useless at telling you about the conditions inside the plane too. If you're rolling and yawing at the same time (known as a coordinated turn), the water in the glass will stay level (relative to the bottom of the glass), and the glass will not slide along its surface, even in a 20 degree bank.