Trish is right, one can see in the water even with poor vision.
My vision is so poor that I have to sleep with my glasses on just to be able to see my dreams. I have snorkeled a number of times and have found that I can see about 90% of all there is to see. The fish stay by the coral, which is very close to the surface in many cases. The only time I missed my glasses is when I thought I saw a horseshoe crab on the bottom of the ocean. I took a photo of it, but later discovered it was a stingray!
If you can get a corrective mask - great. If not - don't be dissuaded from taking the underwater adventure. It is great - even for the bats amongst us!