Today we use "anti-fog flow-coat" on the inside of the lenses. This makes the lenses superb against dew, but also more sensitive to cleaning.
When the lenses get dirty on the inside, you must add moisture such as water or a suitable cleaning agent (Lens Cleaning Set | Sweet Protection) before wiping it off with the glasses bag that comes with it. If you try to dry them clean without moisture, you will drag salt / dirt / dust around the coating and leave marks on the inside of the lenses. This is difficult or impossible to correct afterwards.
Do not use any solvents (for example: Alcohol, acetone) or any other aggressive washing detergents that can change the qualities of the lens.