Well, you MIGHT be able to do that, and you might not. In a real panic situation (power outage closing most/all businesses, people looting stores), it could be very difficult. If you're at all serious about this, you have to be prepared, emphasis on the "pre". A good base stock of canned foods, bottled water, meds and first aid supplies (and realistically arms and ammo) are a must.
I've got all my stores at my house. Just need to load up my truck and head for the boat (10 minutes away, with only one traffic light). You are right though, I should load up with more supplies, even if it only to shack up in my house for a couple of months during a disaster.
I would only pack up the boat and "head out" if I thought my odds were better for my wife and kids out at sea than on dry land. In 99% of the cases you would probably be better on dry land. But the boat is there for that 1% case.