Here’s Sumdood from National Geographic, asking that question: How many water balloons does it take to stop a bullet — a .44 Magnum from a Smith Model 629, to be specific?
If you’re enough of a gun nut to read this site regularly, you’ll be pretty close to the answer, having seen other instances of firing into water. (In fact, it’s how bullets are recovered in crime labs, for forensic ballistics). But you’ll seldom see it given so much of a reality-TV build-up, with the payoff coming in slow-motion video.
The key bit is the slo-mo at the end, where they’re showing the bullet speed in fps as it declerates through each balloon. After the second balloon, the mighty .44 Mag is down to airsoft levels of fps, and it only goes downhill from there.
So why don’t they use water balloons as armor? Well, the first shot lets all the water out, and nobody fires one shot at you these days. Plus, it’s about as heavy as armor gets.