Normal bookbag? Really! That bag doesn't look like any book bag I have ever seen. With its off center water pocket (which will render the bag very awkward when full).

There is a reason all hydration packs have their bladders in the center and close to the back.

Actually, the unique look, logo and all, might be advantageous. The bad guys will recognize it and conclude to leave it alone, since it is filled with junk, and continue on, searching for something better, a not particularly difficult task.

You may fill that bag with whatever you wish, but it is not necessary to do so in order to determine its volume. They give the dimensions are 18x12x6 inches, which is a volume of 1296 cubic inches, around 21 liters (litres?, whatever). That is about half the size of the typical day pack, 2500 cu in. For bigger loads and overnight trips, I prefer a pack at around 4000 -4500 cu in. The typical load would weigh around 45 pounds.

Gear costs money. That's life. You need to choose wisely, and plan for the long term. Fortunately my profession, employer, and my hobbies have all given me a lot of time outdoors and the incentive to acquire decent gear. Most of this is quite applicable to a variety of emergency situations. If I have learned anything, it is to buy quality gear which will function well, rather than cheaper stuff which will fail unexpectedly (typically in a crisis). Fortunately not all good gear is expensive - think Mora knives.

There is always someone who will peddle subpar items at a seemingly bargain price - like this survival lash up. Although the price really isn't a bargain. Like I said, it is better than nothing, but not much better.