Immediately before and during a fight, boxers are often seen taking water, and spitting it out into a bucket. They may be thirsty, especially in the later rounds of a fight, but the reason they rarely swallow it is due to how it feels to be punched to the body with water on the way to the stomach. It’s uncomfortable and can affect the boxers performance. So they may take water to wet the mouth, swill it around to give them some small refreshment, and maybe to wash away some blood which can build up when they are taking punches to the face, especially the nose and mouth.
If a boxer does take a drink, it’s generally no more than a small mouthful.
Boxing Unboxed 