Updated: Aug 8, 2022
When SHTF (Sh*t Hits The Fan) you must meet water needs first. Without water you will stop thinking clearly, possibly make bad survival decisions, and could even die within 3 or 4 days.
You MUST plan ahead: scrabbling around for water AFTER it stops flowing is too late. Running to the store for bottled water--along with thousands of other people who just figured this out--will at least be risky. (Those viral COVID toilet-paper smackdowns were just a warmup--fights over water and food will be way worse.) If you try it, be ready for delays, long lines, bare shelves, and to defend whatever you finally grab. Expect to pay cash, since electronic payment systems will likely be down.
Given enough notice, you can try to fill sinks, pans, bottles and bathtubs with any water still under pressure in your home. Your cleanliness level may make DRINKING this water a bad idea, but it can be used for cleaning and cooking (if you can heat it.) Ditto for water in your hot water heater.
Prior prep is a must, within budget and space limitations. Store as much water as is practical, and be prepared to forage for more. Also be ready to filter and purify, if not boil, found water--including rainwater.
A small apartment or condo may not permit more than a few cases of bottled water. Any amount is better than none, since you will at least be able to hydrate while strategizing your next steps (bug out...where to? hunker down? where will more water and/or food come from?) Water bottles are also easily transported if leaving home base. Water stored off-site (with neighbors, maybe in your building's garage, rented storage space) may not be easy to reach at crunch time.
Apartment dwellers could also look at "aqua pods" or "bathtub bladders," essentially large hot-water-bottle style plastic containers that you fill in an emergency. These can help maintain the potability of any water collected last minute in an otherwise dirty sink or bathtub.
Store water in sturdy, preferably BPA-free bottles with a positive seal. Carefully washed/dried product containers can work. Some preppers use bleach bottles (and bleach residue can help treat long term supplies.)
Space permitting, larger containers or barrels are more efficient--but require more effort. Food grade BPA-free plastic barrels are still readily available online or at local outlets, from 10 to 250 gallons. 55-gallon barrels are most common. Some preppers bury and fill even larger storage tanks. Others note that larger containers are ponderous and hard to manage. You will have to decide. If you are frail, elderly or disabled, use ONLY containers you can easily lift and carry.
While bigger is better, supply rotation can be laborious--even treated water must be periodically rotated and replaced. A 55-gallon barrel of water weighs approximately 482 lbs, tough to handle once filled. An inexpensive electric utility pump, available at most home / hardware stores, will really help to drain your barrels--which can then be refilled using a potable water hose or common garden hose. Flush a new or clean hose with bleach and water BEFORE filling the barrel.
Some recommend freezing water to store it--which could be feasible if you have freezer space (and the power stays on.) Most homes lack substantial unused freezing capacity.
Commercial bottled water should be rotated at least annually.
Treating (disinfecting) stored water
Hoses, barrels, bottles, even the water you use, may harbor microorganisms that could multiply over time, fouling your water supply. Some containers could also leach chemicals into the contents. Water stored long term should be treated with bleach, calcium hypochlorite, chlorine dioxide, or other commercially formulated compounds. See this post from the Provident Prepper for suggestions. Some copper/silver treatments are claimed to stabilize water for up to 5 years.
I still use plain, unscented bleach. This is inexpensive and easy to find. 2 tablespoons per 55 gallons, no more than a teaspoon for 5 gallons.
Some preppers store water untreated, then purify or filter when using it. I prefer not "let things grow" for a year or so before tackling purification. Also, use of food grade containers should alleviate the issue of impurities leached into water over time.
Water after SHTF
Do not trust untreated (by you) open-source water, especially days or weeks after a general societal (and industrial) collapse. You cannot know what "upstream" pollutants foul rivers and streams. These could range from no-longer-controlled industrial waste to urine or feces. Ponds or artificial lakes may have rampant algae or other growths, as well as wind-carried ash, debris and a smorgasbord of all-species excretia.
Remember, water-borne illnesses like cholera, dysentery, or just plain diarrhea may be deadly without proper treatment--which may no longer be available. Sickness could also prevent you from foraging or protecting your family. You simply cannot take this risk.
Be sensible approaching these sources. Grid-down lawlessness will increase risk on easy-access paths to water. Assume looters lurk. Avoid daytime visits, don't go alone (if possible,) be armed and ready to defend yourself and the water you carry. Get in and out fast--don't carry too much water any one trip, and be ready to collect it efficiently (how will you scoop? is the main container easily, quietly filled, rugged and sealable? can you move easily with hands free at all times?)
Back home, "raw" water should be boiled (fuel permitting) and filtered (even through a t-shirt, preferably a survival-grade filtration setup) and then purified with bleach or one of the other compounds listed above. Water purification tablets (often made of the same chemicals) are great for this, and inexpensive .
Planning ahead should prevent, or at least reduce the chance, that you will unthinkingly or desperately gulp down the wrong mixture of chemicals and organic waste--which may look and smell exactly like clean tap water.
Be smart, stay sharp, think ahead--preferably now, while you have time.