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Like OS I always buy the jacket one size bigger.

I do the same but also so I can put it on OVER my PFD when it starts raining on the water. I don't want to have to take it off when a sudden rain squall starts to put my rain jacket on. Your PFD will give you some extra insulation plus having a bigger size increases the air circulation which helps with moisture management when your paddling or portaging in the rain.