Generally speaking, you can survive without food for several weeks but will last only a few days without water.
How much water should you drink per day? The answer varies depending on the season, daily temperature range, and your level of activity. A rule of thumb is to drink water in sufficient quantity to avoid becoming thirsty.
When on the trail, stop at quality water sources periodically to drink and replenish your supply. Try to locate near a water source if you have an emergency and are required to wait until help arrives.
Water in the backcountry must be treated chemically (purification tablets), filtered (pumps, strainers), or boiled (to a rolling boil for at least one minute). Failure to do so risks exposure to water-borne parasites, organisms, and other contamination.
Avoid standing or stagnant water if at all possible. Aim for clear streams or lakes. Clear, cold, rapid streams descending from recent snowmelt are the best sources. Remove debris and visible items prior to purification.
Boiling is considered the most effective, but is often impractical due to the resources needed to heat water (cooking fire or stove and fuel, metal boiling container). Stove fuel may be limited, and wood may be in short supply.
A fire may be unsafe or impractical due to fire danger.
The other options are more easily carried, and much quicker to deploy.
In-bottle units are reasonably compact, and can be used very quickly. They also can be used to carry water much like a water bottle (this is my personal preference).
Pumps and external filters do a fine job, but are bulkier than necessary for personal use.
Very lightweight, usually supplied in a small bottle or container. Slower to deploy than filter methods, and have a chemical taste. I carry a bottle as a backup to the in-bottle filter.
Let’s make something very clear. If you are in an emergency survival situation, and have no way to purify drinking water, drink the water anyway. Your life may depend on it. Try for the best water source (see above). Consider straining the water through a rag or cloth first.