I always dilate the last month or so of my pregnancies, so we talk about being induced when I get to about 6cm. My water has never broken on its own, the doctor has done it both times once I'm in the hospital being induced. He stripped my membranes last time and my water still didn't break. I've known several women who have gotten out of bed or sat down to tinkle and it broke, but no one ever talks about a huge gush of water. They say its more like a trickle. When pregnant with me, my mom's water broke at work, but she just thought she was having issues with bladder control at first.
I'm pretty sure I'd be embarrassed if I had a huge gush of water while out shopping or something, but I've never heard of it really happening.