Spit washed off with the water your diving in does prevent steam ups.apperently you can get something that divers use to rub on your glasses lenses to stop them fogging
thats the stuff works a treat....sainsburys also randomly sell a lens cleaner spray that works as wellIf you do get trouble, you get use visor spray for motorcyclists (bob heath's I think), but just keeping them clean (ahem) prevents a lot of fogging.