So far I've not noticed the river in Riverview to totally freeze so have fished there in winter, the lake in Twinbrook did though, it sort of makes sense as flowing water would be less likely to freeze solid unless the temps were extremely cold. I would assume the sea doesn't freeze either else how can we get the polar bear club thing they mentioned for swimming in winter?
I'm currently aiming to get level 10 fishing before winter to get some death fish in my current game, and not just winter as a hard frost freezes the fish hatchery, but if it behaves as I've seen in the past hopefully the river wont freeze.