after the swim
You do get very cold afterwards, though. Not the first five minutes, but once I have changed into dry clothes, I start shivering. I should probably go for a fast walk then (instead I went to to buy the paper and talked incoherently to the people in the shop -being cold does that to me).
Swimming in the sea is the best boost I can think of. While I'm in the water, I forget about everything; it makes me feel strong, cleansed and energetic. It's almost impossible to have a bad day after that.