It depends on how nearsighted you are. If you have something like -5 you'll still notice being nearsighted, whereas someone with only -1.5 might suddenly experience seeing perfectly.

It's because the refractive index of water is different than air. Also, the pressure has an effect on the eye, changing the default focus. Finally you'll also have a more narrow vision from goggles or from squinting, which itself helps focus ever so slightly.

If you're in a pool there's not a whole lot of details to look at to begin with and we're not terrible good at judging distances under water anyway. So the whole experience is different and difficult to compare to normal vision.

If you do get goggles with lenses you might want to choose a correction that is a few points under your normal glasses because the water gives you one for free.