A physics answer
As a Doctor of physics, I can tell you that you are short-sighted, because when you are in water, your eye lenses become less effective. the fact that they give about the right result then means that in air they will be too strong, making your eyes focus somewhere about a hand-span or two beyond your nose, instead of at the far horizon.
Your optician/occulist can give you a more professional diagnosis, but between Arrell and I we could probably work out what sort of lens you needed...