Why do divers spit in their masks?

The best way to get rid of or prevent fog is to decrease the surface tension of the water droplets that would otherwise become a foggy field. Rather than be able to stick to every little micro-indentation, the condensed water will instead form into larger droplets. These droplets can simply roll off of your mask.

Solutions that can break up and lower the surface tension like this are called surfactant. Saliva can act as a surfactant, and that is why divers spit into their masks. While there are many other options that you could use instead, most divers would say that nothing is as readily available or as free as spit for this purpose.