In hot weather one should have water, be hydrated, and know when to quit. Hot weather will make one sweat more and will be intense on your body.
At the same time it goes the same for cold weather.
Olympian swimmers train in cold water, because it slows the muscles, causing the person to work harder so when they are in normal temperature water that is not as cold they can go faster. At the same time Olympian sprinters train where it is highly elevated and warm because the elevation lessens the oxygen flow, and the warm air will help them in any temperature.
Both are extremes to exercise in, but done correctly, both can be beneficial.
I hope this helps :) Please let me know if you have further questions!