New Scuba and Snorkeling Masks!
New scuba diving masks have residue left over from the manufacturing process on the lens. Unless you remove this coating from the inside of the lens, you will find your mask constantly fogging up no matter how much defogging agent you use. There are two tried-and-true methods to remove the residue.
The Toothpaste Trick
Squirt toothpaste on the inside of the lens and rub it around with your finger or a soft cloth for 10 minutes. The simpler the toothpaste, the better, so try to find a paste without bleaching agents and confetti.
It may help to leave toothpaste on the mask overnight or to scrub the mask several times to allow the chemicals to react. Avoid using an extremely abrasive toothpaste or rough cloth, as these can scratch the inside of the glass.