I can empathize with the brrr chilly cold feeling even in a heated pool. I used to swim on a team year round - outside - in sunny So Cal. I used to turn blue. They kicked me out of the pool every work out into the jacuzzi and eventually told my mom I couldn't swim during the winter with them. I get it. Even when you are working hard, you get chilled. You could try a wet suit, but that would give you so much flotation that you would pretty much float from wall to wall with very little effort in a 1/2 olympic sized pool (that's 25meters across, right?). Maybe you should consider other gym activities in the colder months - or, as you are doing, swim during the warmer part of the day.