At what point is it too cold to let my GSD/Lab mix to swim in the water?

My 5 year old lab/GSD mix loves playing fetch in the water. It’s one of his favourite activity to go on a walk at the waterfront and then go play in the lake. I know labs are natural good swimmers, and do well in water since they were bread to help out fishermen. Currently the outside temperature is around 5C, and my gut tells me it should be fine since he loves playing in the snow in -25C weather, but the water may be a different story and I wanted other people’s opinion. Thanks