The reason why water causes steaming and clouding of vision, is because of the scattering effect of single droplets. Due to water having a high surface tension on lenses and due to its refractivity, when water vapour condenses on the surface of the glasses, it forms miniscule droplets of water, each being almost completely spherical, acting as lenses, refracting light that enters the droplet through various angles. This causes the end result to be an opaque-translucent white surface.