I bought a prescription snorkel mask on
Amazon and couldn't be happier. For the purposes of snorkeling, you don't really need a perfect prescription. Getting the prescription right is important for reading small text and so forth, but you can get away with an approximation in a mask.
Here's the model I got:
http://amzn.com/B00D9FPU5S
I also got these prescription goggles for situations where I didn't need a full mask (i.e. in the pool or on a waterslide):
http://amzn.com/B001MWRSAA
To pick the power, look at your prescription. For each eye there will be a spherical power and a cylinder addition (plus an angle, which isn't important here). For example, my left eye is sphere: -9.125, cylinder: +1.75. That means I have a power that varies from -9.125 to -7.375 (you add the cylinder to the spherical to get the other end of the range). For snorkeling I can take anything in that range, so I just picked -8, which works great. People with no astigmatism don't have a cylinder range, so you just round the sphere number to the nearest available off-the-shelf number.
Interestingly enough, my previous doctor wrote the prescription as Sphere: -7.375, Cylinder: -1.75. It's actually the same prescription (-9.125 to -7.375). It's just optometrists tend to write it differently than ophthalmologists.
Off-the shelf lenses are typically in increments of 0.5, so I could probably use -7.5, -8, -8.5, or -9 and be fine, but the middle of the range is a safe bet.
If you're not sure how to interpret your prescription, feel free to post the numbers here and someone will surely be able to recommend an option.
If you have very different prescriptions between the two eyes, you'll probably need fancier googles with snap-in lenses, or custom-made ones (at which point you might as well go for fully-specified prescription lenses, with astigmatism correction and everything.