You go out to sea and you forget to treat the lens of your mask with anti-fog treatment, or the lens still get foggy after it?
There is a last minute quick remedy, at least to best solve the problem immediately. You need to find something abrasive that can take off the silicone and glue that have deposited on the lens during manufacturing process of the mask. It will be sufficient to use some rough algae taken from a rock or the pier of the harbor and rub it in the internal part of the lens and…problem solved! Maybe it will not be sufficient for a 100% perfect job, but it will surely save the day!