HEretoday...have you heard of the solar blankets...thatarea very efficient and ecofriendly way of heating swimming pools...they can actually raise the temperature of the pool upto 10 degree F.. Do see the link to find out more...http://www.royalswimmingpools.com/solar_blanket.htm
I would say that it can be physically dangerous to be in a pool that is too cold (hypothermia dangers.) I guess that you should heat it, but just enough to take the "chill" out of the water.
My parent's pool sported a solar blanket, and it worked quite well on very hot, sunny days, but as the sun got weaker, so did the water become cooler. Those blankets are quite useful, but only to an extent.
We have a blogger job opening at EcoFriendlyDaily.com and GadgetAddiction.com, if you're interested please email us with samples of your work and why you would be a great addition our team!