Be wary of drinking water straight from a river, pond or lake. Unless you’re desperate, you should assume that these sources of water are teeming with bacteria and parasites.
Without water of any kind, you’re doomed.
There’s no way around it. Dehydration can rapidly affect the body without an ample water supply. Depending on the person’s physical condition, environment and exertion levels, they might notice changes in their body after just a few hours.