Here's the scoop.
Obviously even 50m (over 150') is far deeper than what most divers will ever do in their lifetime.
The problem lies with how watches are rated. They are stuck into a pressure jar compressed, sitting there on the bottom. They aren't touched, they aren't moved around, and they aren't played with. This is a
hydrostatic pressure test, meaning the water is still and doesn't move.
Basic physics, however, makes it clear that the second you move a watch under pressure, you greatly increase the pressure the watch is getting on one side, while decreasing it on the other. So essentially, the second you move, you have just multiplied the pressure by many times. Now hit it on a rock at 90'. Imagine how much pressure there must there there!
The reality is even though less than 1% will ever actually go to 100m, let alone 200m, you need a higher rated watch for scuba because of the stresses and movement it goes through at depth, which are very different from the stresses the watch got sitting at the bottom of a pressure pot.
All that said, I know a number of divers that squeak by with 100m watches. They do so with full realization that the watch could go at any time, however... so I wouldn't recommend doing this with a watch you actually like and can't afford to replace.
Another thing to consider is the buttons. Watches rated to 100m often do not have buttons that can be actuated underwater. 200m watches are designed so you can push the buttons underwater and still maintain the pressure seal.
Long story short, unless you are a starving student who literally can't afford the minor price jump of a 200m watch over a 100m watch, you'd be better served with a 200m watch if you're intending it for scuba.
FWIW, I recently picked up a 200m Casio G-Shock for $45 at Walmart. They don't have to be expensive to work well as dive watches. (It's not my regular dive watch since I got my Citizen, but I've used them with great success in the past.)
You may be better off still with greater than 200m is you plan to start extreme depth technical diving, and are considering just how tough those records are to break.
Btw: There's no such thing as water "proof" anymore (although there could be some 5-figure high end watch that still makes that statement). They're all water-
resistant to various depths.