This is a maintenance issue and you should be draining your tank empty each year. This will help keep sediment out of your tank and calcium build up. Both of these issues cause the water heater to fail prematurely and lower the quality of the drinking water in side of them. So you need to start doing this each year it will honestly save you money, since your water heater will last much longer.
Then if it is ever needed for emergency drinking water, it will be much cleaner with no sediment in the bottom to muddy it up. Mine is a 40 gallon water heater, that will keep a family of 4 healthy and happy for 10 days. It could be stretched and keep you all going for much longer, 15 days safely. That depends on the weather and your physical output. If you are doing physical labor or it is very warm where you are, you will need a lot more water to survive.
For most survival situations this will be enough water to get you and your family through til help arrives. If it is the start of something much longer then it will give you a head start and a place to find untold gallons of fresh drinking water. Most will overlook their own hot water tanks and head out in search of fresh drinking water, you will have the knowledge to sit tight and move from house to house and reap the rewards of your knowledge.
Here is the process to drain your water heater each year, the instructions work for getting the water out for drinking as well. The only difference is you would fill a container and shut the water off until more is needed for drinking. In the case of draining the tank yearly you just let it flow out into the yard or garden until empty.
Warning the water will be very hot!
Turn the gas to the pilot position or flip the breaker to the off position if your water heater is electric. This will insure the tank is not heated while it is empty. Then near the bottom of the tank you will find a valve, it looks similar to a hose bib. The thing your screw your garden hose onto, except they normally do not come with the little handle on top to turn. So get a piece of garden hose either to fill your container for drinking, or to empty the tank out into the yard or garden.
You need to turn the water valve off that is on top of the tank, normally only one side of your hot or cold has the shut off valve. Now you can open the valve at the bottom, normally you would use a flat screwdriver. But some models might need a pair of pliers. The water will not flow yet because the water heater is sealed and we need to let air in so the water can flow out. You should have a pressure relief valve, you can flip the pressure relief valve open. You should hear air entering the water heater and water should start flowing out the hose. If you do not have the relief valve you will need either a wrench or heavy pliers to loosen one of the nuts holding the water lines on top of the water heater. You do not need to remove the nut, just loosen it until the water starts flowing.
When you are done draining the tank, tighten the nut on top holding the water line to the hot water tank. Close the pressure relief valve. Now turn on the water supply on top of the unit. Let the water flush out the tank for a few minutes. Now you can close the lower valve at the garden hose. Remove the garden hose, go inside the house and open each hot water tap, kitchen, bathroom, and garage if you have a sink there. It will be spitting air out for a minute or so, then close all the faucets. Back out to the garage and turn the gas or electric power back on.
Make sure to check all the fitting you touched and even the fitting you did not open or loosen. Make sure there are no leaks. This gives you many more years of life out of that water heater and now you know where to find water in an emergency.
Forty to eighty gallons of water can make the difference between surviving and not making it, this is a top skill to have in your skill set. You can move from house to house collecting water for the long term. Of course you would only collect from empty houses, but many people will not think to use the water from the hot water heater.
Take a look at the back tank on all the toilets as well while you are looking for water. I feel you could drink this water from the holding tank on the back of the toilet without bringing it to a boil. But if you have the capability to bring that water to a boil, do it just to be safe. You can decide what to do with the water in the bowl, that water is not in a clean environment. So bringing it to a boil is a minimum and unless you are in dire need most will not knowing drink that water. I guess that depends on how thirsty you are. One more way you are learning to be a survivor.