By combining direct action on the situation of the pool (protection from wind, optimal exposure vis-à-vis the sun?) By adding a suitable heating equipment and protection (to prevent heat loss ) you will get the best results.
Cover swimming pool at night with a blanket or isothermal rolling shutter you will keep up to 3 to 4 ° C heat. Beware some hedges do not meet safety standards.
Various facilities are available to heat your pool (heat pumps, heat exchangers, heaters, and solar)