In an emergency, you may or may not have access to your water storage. It's a good idea to know about the different ways that you can find water in an emergency. Here are some possibilities:
1) Tap Water - In many scenarios, tap water may still be flowing and pure. Listen to authorities for instructions as to the purity of your water. When in doubt, boil it.
2) Home Storage Water - Now is the time to use this water. If you can't get to it, maybe you'll have access to water in 72 hour kits. I also keep a case of water bottles in the back of my car. We use it all the time for soccer games or road trips. Even on the run, I'll have access to a little bit of water.
3) Home Water Sources - You have more water in your house than you thought. Your hot water heater (turn off gas/electricity to the tank first) and toilet tanks are just two examples. Often there is water that lingers in our garden hoses or sprinklers. A waterbed is also a source of water (if you haven't added fungicides) and probably your freezer has a small supply of ice. You can also extract water from your pipes, but will need to shut off your incoming water supply first (as to not contaminate). Then turn on your lowest faucet to access the water. The water from a swimming pool can be used for cleaning and flushing toilets but not drinking. Even if you're unsure of one of these sources drinkability, you can always use the water for cleaning. I wouldn't count any of these sources towards your home storage. They are unpredictable at best.
4) Outside Water Sources - You should never drink flood waters. Nearby streams, ponds or rivers can be a source of water, but you should always take measure to make the water safe by straining, boiling, distilling or chlorinating it.
Here are some great links for emergency water: