Hot Water Heaters

How to Get Emergency Drinking Water from a Water Heater
http://www.wikihow.com/Get-Emergency-Drinking-Water-from-a-Water-Heater

Image A typical home water heater can provide between 30 and 60 or more gallons of clean drinking water during a disaster. Hurricanes, floods, earthquakes, and other power outages may prevent you from having many things, but clean drinking water is not one of them. To reclaim some clean drinking water from you water heater, and to tap your inner MacGyver, this is what you'll need to do. ---CONTINUED at LINK---

From the LDS Preparedness Manual:

Page 110

Some people have plumbed old water heaters or other tanks in line with their cold water supply to add an always rotated source of water. Two cautions are in order: 1) make sure the tanks can handle the pressure (50 psi min.), and 2) if the tanks are in series with the house plumbing, this method is susceptible to contamination of the municipal water system. The system can be fed off the water lines with a shutoff valve (and a second drain line), preventing the water from being contaminated as long as the valve was closed at the time of contamination. Water can only be realistically stored for short-term emergencies, after that some emergency supply of water needs to be developed.

Hidden Water Sources in Your Home

Emergency Water Storage
http://www.nationalterroralert.com/safewater/

To use the water in your hot-water tank, be sure the electricity or gas is off, and open the drain at the bottom of the tank. Start the water flowing by turning off the water intake valve and turning on a hot-water faucet. Do not turn on the gas or electricity when the tank is empty.

Use caution when utilizing your hot water heater for drinking/consumption. Your hot water heater only heats to 120 to 130F, and this is NOT hot enough to kill a virus.
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Read this:

https://www.families.com/germ-laundering

According to the CDC (Center for Disease Control) in order to be effective in killing germs laundry (linens) needs to be processed at a minimum of 160 degrees Fahrenheit for at least twenty-five minutes. I don't know about you but my water heater is set to not get hotter than 120 for safety reasons.
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Consider additional purification, as necessary.

Drinking Water Available Immediately
https://grandpappy.org/index.htm

(Snip) ... If you are at home when the emergency occurs, then immediately check to see if the faucet water pressure is still on. If it is then fill every possible container in the house that will hold water and not leak. For example: pots, pans, plastic containers, drinking glasses and drinking cups, bowls, and every sink and bathtub.

If the water is off, you still have three sources of clean drinking water:
1. Ice cubes in the freezer, and
2. Water inside your water pipes, and
3. Water inside the hot water heater.

Even if the water pressure is off, ---CONTINUED at LINK---

Survival Water From Your Water Heater
https://www.askaprepper.com/survival-water-from-your-hot-water-heater/

  (SNIP) ... most of us have a number of water supplies readily available, within walking distance of our homes. We also have clean water in our homes, ready for our use. All we have to do is find a way to access it.

The water heater in most homes consists of a 30 to 50 gallon tank, with a heating element inside. This tank is always full, even though you may not have water flowing through your home’s pipes.   ---CONTINUED---

Resource Tipsheet: Water Storage
https://extension.colostate.edu/disaster-web-sites/water-storage/

Scroll down the page for:

Emergency Sources of Water

In an emergency, if you have not previously stored water and commercial or public sources of water are not available, drain water from your plumbing system. Unless you are advised that the public water supply has been contaminated and is not safe, open the drain valve at the bottom of the water heater and salvage the water stored in the heater. A typical water heater holds 30-60 gallons of water. Discard the first few gallons if they contain rust or sediment. Let the water heater cool before draining it from the heater so it does not scald you. Turn off the electricity or gas to the water heater to prevent the heater from operating without water. Once water has been drained into clean, sanitized containers, add 5-7 drops of chlorine bleach* per gallon of water, and stir or shake the solution to mix it. Let it set 30 minutes before use.