I wear sunglasses every time I paddle, I love polarized lenses (those Orosi ones were awesome) but have moved to lighter tints cause you can see better in mixed/low light and see through the 'fog' more easily.  
Dark glasses are best for open water; CO or CA boating and where you dry off between rapids.  

Rainex won't stop fog, but it makes the water sheet off quickly and keeps hard-water 'haze' off your lenses.
Clay