One of the less mentioned situations is when ppl use sunprotection lotions or have just a very oily face that gets washed off and them deposited on the inside of the mask and cause it to fog up.
Baby shampoo workouts best for me for years. Most important is do it right before a dive and put it on. I see too many divers who defor the mask 20min before and wear it on the neck. And have a foggy situation 5min down the dive
Growing up and Snorkeling in Lake Erie I was told to use seaweed. Not sure if it's name but plentiful on rock or structures near it just above the surface Its deep green a very soft fine. Great to wad up and chuck at someone. It sticks very well ;)
I dive with a full beard, for me a lot comes down to the mask fitment, I try to keep my upper lip shorter 7 to 10mm. Was on a live aboard in the GBR, and forgot my razor ended up using Vaseline (or silicone grease should work) around my mask line, but I tend to try to keep it tidy when I can. Read somewhere that shaving 2mm under your nose might do the trick as well.
There is some truth in there. But because we're diluting a few drops and only using a spray of that each time, there's only a tiny amount that ends up in the ocean, that further dilutes it. Compared to what you use in a shower that ends up in the ocean or sunscreen it's a tiny amount. It is important that we limit what ends up in the ocean and there are some reef-safe defogs out there.
In California it's common to use a torn off piece of kelp rubbed on the inside of the lens - works well even after you're in the water. Defog-film reminded me of some glue-in bifocal lenses I used before I got prescription lenses. Without a good mask, diving is kinda pointless...
