When you buy without ordering the Rx, they only include the diopter. I can't say how much they vary them, or whether you can get different strengths in each eye, since I haven't looked into it. Custom Rx go to .25 diopters, I believe. They may only carry masks in .50 increments, just trying to get close.
Depending on what your prescription is this may be fine. If you have astigmatism, that is not handled with this approach. If you want that included, you need to order a specific Rx. As benchill mentioned, in the water things are magnified, so having a slightly off prescription is not as big a deal as glasses you're using every day, driving, etc.
All comes down to you, your prescription, and what it costs. In my case, I found that ordering the custom prescription made it cheaper thanks to insurance. I have moderate astigmatism, all things being equal I chose the custom Rx over just the diopter. I'm bad enough that I can tell the difference of the prescription mask in the water (compared to no correction, not an off the shelf semi correction), but it is very nice for entry/exit, finding the boat, etc. You just have to look like a knucklehead walking on the boat/beach etc. with a mask on.
At least I didn't have to wear it in town!

That was a funny story benchill, missed it the first time.
And, FWIW - as others mentioned, I ALWAYS carry spare glasses... now... been there done that and learned from my mistake.
irbgolfin