You've heard you can survive three weeks without food and three days without water. The second half is the one that matters. In heat, exertion, or illness, dehydration becomes dangerous in hours, not days. Water is the first category in every go-bag for a reason.
Plan one gallon per person per day for drinking and basic hygiene. A 72-hour kit means three gallons per person, which is heavy and bulky. That's why the kit carries purification, not just stored water: the ability to make found water safe multiplies every source you encounter.
Carry a primary purifier and a backup filter. Our primary pick is the GRAYL press purifier: fill, press, drink in seconds, and it handles viruses, not just bacteria. The backup is the Sawyer Squeeze: three ounces, near-infinite lifespan, screws onto any bottle. If one fails or clogs, you're still drinking.
Boiling works but needs fuel, time, and a vessel. Chemical tablets work but need wait time and taste terrible. A press purifier plus a squeeze filter covers nearly every scenario with no fuel and no waiting. That's the whole water plan.