One of mine went out for a mid winter hike with us when she was maybe 10 weeks old. It was about 20 F and she dove in the water and swam around repeatedly. We were a little horrified that it might be bad for such a small pup to be in such cold water. She was unphased, Throughout her life she seemed to chose cold days to swim when hiking near water. Hot weather she only swam when asked to fetch something.
On an sadder note my current pup laid down on her second morning walk today and refused to move. We took her to the vet and she had surgery to remove her spleen. It seems unlikely she will reach her 11th birthday in May if the tumor was malignant and most spleen tumors in dogs are. We are awaiting pathology results. I can't imagine being without her.