In order to have a comfortable snorkeling experience, you don't have to, but you should shave your mustache. A great mask will form a seal around your face to keep water out, but any hair that stays in your mask, it doesn't matter how good-fitting the mask is, it will start leaking if you have a mustache. Water will end up in the nose area of the mask, which for some may be uncomfortable, even though you don’t breathe through your nose during a snorkeling outing but through your mouth hence why a snorkel is used.
Those who are unwilling to sacrifice their mustache to go on a snorkeling adventure will have to either ignore the discomfort of having water in the nose area or will have to continuously lift the mask to let the water out. Lifting the mask too often is not recommended since it can cause it to fog, and being able to see everything at the reef is why you go snorkeling in the first place. It is recommended to lift the mask the fewer times possible so that fogging doesn't happen during your snorkeling experience.
Saying goodbye to your mustache may be hard, especially if it has been part of your image for a long time, but it is worth it in order to eliminate any problems or discomfort while swimming in the ocean over the wonderful coral reefs. Keep in mind that if you are a beginner, it is always best to go on a guided tour. The guide will share with you tips and tricks during the tour and will help make the experience as comfortable as possible.