DiveDiversions
Registered
Yes, water slightly magnifies so will alter prescription glasses.Tried this too. Works, somewhat. Keep in mind that because there is water between the lenses and mask the refraction will cause the prescription to no longer be correct.
In the case of just needing readers then trial and error as suggested above can work.
However with full script glasses T&E aren't an option.
We have have had one user that went to their local thrift/charity stores and picked up some glasses they found that was close to their prescription for ~$2 each and tried some until they found something that works.
For reading glasses, make sure you keep the arms and screws so you can reassemble and exchange them as it can take a bit of trial and error to find the correct pair.
It's a cheap flexible option and not suited to everyone.