Whilst recently on holiday in Southern France the campsite we were staying at had a fun pool complex plus an unheated pool. The unheated pool was a standard rectangular shape ideal for swimming lengths, whilst the other pools were obviously not designed to actually swim in.
Initially I steered well clear of the unheated pool but when I wanted to burn off some calories I ventured in. The air temperature being in the low thirties , my dipped in toe indicated that the risk of hydrocution (sudden death by submersion in cold water - very rare..) was high in my mind. Once in and swimming lengths I discovered that this was far superior to swimming my lengths in my local heated pool at home. Most pools in the UK have to make a compromise between being comfortable for people standing around playing and those wanting to swim lengths. Personally I find after a while in my local pool I am starting to feel warm, however in the unheated pool I felt completely comfortable, not warm not cold.
I believe that in pools dedicated to swimmers that they drop the temperature by a few degrees. I suspect somewhere a scientist has calculated the optimal pool temperature for swimmers, however if you get chance try an unheated pool at natures preferred temperature just jump in and give it a go. You never know you might just prefer it.