* A glass of warm water: This emphasizes that the water is warm. It specifically mentions the quantity and temperature.
* Warm water: This is a more general term that simply refers to water that is warm. It doesn't specify the quantity.
The best choice depends on the context. If you're talking about a specific amount of warm water, then "a glass of warm water" is the better choice. If you're talking about warm water in general, then "warm water" is more appropriate.