So you're having trouble with your mask fogging? There are a few reasons why this could be happening. 1. If you have a brand new dive mask and haven't prepared it properly by removing the internal silicone layer, it will fog every time you dive. 2. If the lens on your mask is dirty, this will cause fogging as well. 3. If your body temperature is warm and the air temperature is significantly cooler, this can also cause fogging.
As soon as you get a brand new dive mask you should be looking to remove that silicone layer inside the glass lens. Note: this doesn't work on plastic lenses. The best way to remove that layer is using a product such as McNett SeaBuff which is a gel with a very fine abrasive which actually removes the silicone as you rub it into the mask. It's best to do this to a brand new mask 2 - 4 times before you hit the water. The last thing you want when you're out there ready to hunt some fish is for your mask to fog, so it's definitely better to be on the safe side here.
Some people will actually use toothpaste as well, which does work, however, we don't recommend it as it often gets stuck in difficult areas within the mask, then when water gets in your mask, the toothpaste can end up stinging your eyes, which isn't a pleasant experience at all.
If you've already prepared you mask adequately and ensured it's not dirty but it's still fogging, there are a couple of things you can do. Firstly, before you dive, you can try spitting in your mask, onto the lens and giving it a good rub around the lenses. This a solution used by many divers and tends to work quite well. If that's not working for you, you can look at using one of the many anti-fog products on the market such as Frog Spit.