Snorkeling Tips: How to Prevent Mask Fogging During Snorkeling

Prevent Snorkel Mask Fogging In Four Steps

1. Clean your mask often, and then don’t touch it inside. Clean it with toothpaste, and a toothbrush, not your finger. Apply a small amount of non-abrasive toothpaste, and scrub it in hot water. Rinse it completely in hot water. Do this often.
2. Use anti-fog every time you snorkel. The best is a very mild solution of baby shampoo and water, or an anti-fog product. Spray it in your mask, swish it around and dump it out. DO NOT RUB IT AROUND WITH YOUR FINGERS! Your fingers are oily and dirty. Also, don’t rinse it out with fresh water. You want the mild soap coating on the glass. It helps prevent moisture attachment. That is why you want it to be a very mild solution without perfumes so it does not burn or irritate your eyes.
3. Get as little water in the mask as possible. Shake out any anti-fog or other water drops before using it. Try to have a relatively dry face. Once the mask is on with a good seal, try to keep from removing it and allowing moisture to enter. Try not to clear the mask by breathing inside of it unless you must (that adds moisture). We enter the water with a dry mask, over a dry face, and do not take that mask off at all during the snorkel, if possible. And it works.
4. If all else fails and you are out on the water with a foggy mask, remove it, spit in it, shake the mask around to coat the glass (don’t rub it with your fingers), dump it out, and put it back on, this will generally fix the problem. If you don't have a lot of spit, dilute it with a small amount of salt water to coat the glass.

Source:

http://www.tropicalsnorkeling.com/prevent-snorkel-mask-fogging.html