I congratulate you for thinking "outside the box" (or bottle), but IMO, I'm not sure if there is much of an advantage to doing this as opposed to buying bottled water. For one, those vacuum sealer bags aren't that cheap, and they're not easily reusable or reclosable. Plus they don't stand up on their own, so once you open it you'll have to hold it upright. Second, I don't think those bags would be nearly as durable as a plastic bottle. Even double bagging it, it just feels like it would a sharp corner would puncture or wear through too easily. Third, I would question the cleanliness of the water, even after boiling, purifying and sealing. I just wouldn't trust it as much as factory bottled water.. I'm sure the water companies have spent a lot of time developing a safe, durable, and cheap way of storing water, so in this case I think they have probably come up with the best solution so far. Just my opinion.

I think the vacuum sealing idea would work on other items, things that need to be waterproofed, but not accessed on a regular basis. I have always wanted to vacuum pack the items in my first aid kit (like band-aids, medications, gauze, etc), or other emergency items like batteries and matches. Cut a small notch in one side, and it would be easy to tear it open when you need it.