Unlike many questions in the watch hobby, there actually seems to be a clear-cut answer here. In order to be considered a true “dive watch,” the timepiece in question needs to come with the following things: passive illumination (or lume), a diver’s bezel, and at least 200 m worth of water resistance.
Lume is a common feature on a multitude of watches. We call it passive because rather than having to press a button to light up the dial, the hands and indices will give off whatever light they’ve absorbed from your surroundings. So, any time you look at it in darkness, there should be at least some measure of glow. Whether you’re underwater or just have your hands full in general, this added legibility is a vital touch.
Then there’s the bezel. We’ve talked about what they’re for above, but timing your remaining oxygen isn’t the only thing they’re good for. You can actually use it to do math or just set a reminder for when your pizza should come out of the oven.
The water resistance is pretty self-explanatory. If you’re going to be diving, you need something that won’t swamp or short out the moment you break the surface. Still, watches with as little as 50 m are often considered OK for swimming.
But with 200 m to play with, you can rest assured that there won’t be a problem. And because seals do degrade over time, the additional rating means your timepiece will stay waterproof for much longer than something with a lower rating.