The high temperature for a certain day is irrelevant to 'ideal swimming conditions' if the high for 2-3 weeks is always 80-90 and the low never drops below 55-60 then it may be comfortable for swimming. But if it's been average highs in the 60's-70's and lows in the 40's-50's for the past week and you get one 83 degree day, your gonna freeze your **s off trying to swim.
Yes, you can swim outside, when the temperature is 83 degrees. If you are swimming in a pool, the water might not be warm, but it will still feel good. If you are swimming in a lake or river, the water could be cold. Your body has to get used to the temperature and then you can enjoy yourself.
83 degrees is a good temperature to swim. However, if 83 is only the high for the day, make sure the temperature actually is that warm, so that your toes don't get frostbitten when you try to get in the pool. Hope this helps!