No. It depends on the swimmers – and how much you want to spend on the energy bill!  The cost of heating increases twofold every 5°F because the differential between the water and air temperatures increases. Most outdoor pools will get solar gain and most have a temperature of about 80°F or 27°C.  A floating cover on the pool at night will greatly reduce running costs and shell insulation will also be of assistance for outdoor pools and is now required for indoor pools.