Thank you.
You would have to stay in long enough to get quite hypothermic before the implants would have a temperature change. These implants are stored at room temperature, which is way cooler than the implants would possibly get while you are swimming. Not a worry. Best wishes, and thanks for the question.
Enjoy your swim! Cold water will not hurt your implants. You didn't say how long ago you had your Breast Augmentation. Be sure to follow your Plastic Surgeon's advice regarding exercise.