Sunlight would be the cheapest, if we had a lot of it........ But a friend of mine uses an overnight heating system which works by heating blocks on the underside of the pool, and it works really well. But it would have to be installed before the pool was built........ Are you sure there isn't a heating system in place already, as it could be costly to instal?????? Or have you infact not built the pool yet???????
invite the local primary school to come round for a swim and give them all a big drink of pop before they get in. 10 mins later, they will all warm it up for you!