I typically have three mags on my AR-15 carbine: One in the well, one in the ready mag, and then one on a buttstock pouch. That's usually enough to take care of all those maurauding water bottles out there!
My experience with Velcro is that it's great stuff, but here in Florida with all the vegatation and mud, it gets gummed up real fast. Works well in the desert, though, from my experience.
... and that's where we find most of those renegade water bottles out here in Utah.