It is important to understand that no rented mask will fit you as well and be as useful to you as one that you buy yourself and bring with you. ... If you are sure you will never use one again then rent one here in Vieques.
If you wear prescription glasses then it will be difficult to see the corals and fish with a rented mask. You can order a mask made with your prescription in it ( before you come) that will enable you to see well underwater. The alternative to that is to use contact lenses.. . bring them with you.
If you have a mustache the mask is going to leak, some divers use Vaseline on their mustache to make a tighter seal. Men who dive a lot and have mustaches sometimes shave just under the nose to make a tighter fit.
Clean your mask out with a strong detergent or toothpaste to prevent it from fogging up.
More important than any other advice: If your snorkel gets water in it or anything at all makes you panic. Take your snorkel out of you mouth. Take it out! Breathe without it then put it back on!
Snorkeling equipment rental for about $12. per 24 hr day including mask, snorkel and fins. Blue Carib in Esperanza and Blackbeards in Isabel II.