I wear mine to ride down the local dirt jumps so I don’t get laughed at by the kids. In fact now I am cooler than all of them, in their garish motocross getup. It’s a 3 mile ride there, and my arse gets wet from the grit/spray off my back wheel when it’s raining. They do shed water very well but I’m not sure they have much of a “hydrostatic head”, to use the parlance of our times. So if they have a layer of snow sat on them I don’t know how waterproof they’d be.
Sizing wise, I am a bit fat now @ 5’10” and 13st, and just about squeeze into a 32. The legs are quite wide and flappy, so for cycling I don’t think they’d be much fun without a bash. Means I can get my Kyle Straits on underneath without any fuss though.