Regardless of where you get the water (I would NOT use the 15 gallons in my water heater even if I could get them out--electricity loss means water loss in my neighborhood). you need large quantities in ready supply.
CERT members think in terms of a 3-day disaster. That means water before food. We recommend 1 gallon per person per day. For the non-human members of the family, you may need a bit less (a wolf-dog hybrid may need 2 gallons).
Most of us keep gallon jugs and 16-oz bottles handy. I can go for more than two weeks without an outside source of water.
If you have water and are not excessively active, the food issue can be a simple as Meals, Ready to Eat (MRE). While MREs are too high in carbs for long-term survival, they are the right thing for a 3-day emergency.
We stress a 3-day survival kit for every member of the family. The FEMA CERT websites have good starting lists--personal hygiene is usually missing.