We always take our own. You can get a fairly good set of goggles and prescription inserts (that would act like bifocals) for about $60. That way they are the prescription you need, not what they happen to have. Try Sports Chalet, a really good (big) sporting goods store, or a dive shop.
i'm just a bit confused here, how can you rent a prespription snorkel mask from the ship? how do they know what your prescription is? Are they like the glasses you buy in the drugstore? please educate me. i don't wear my glasses when I go snorkeling, but then again i can see enough without them that i can do without them for a few hours.