An old work colleague did something similar actually!
He started off with the pool and an electric heater built for it but found it barely warmed the pool, so then added another. This made the pool bearable but cost a fortune to run!
He sank the pool half into the ground with a layer of insulation underneath then built a deck upto the top of the pool to hide a wrapping of insulation, this reduced the cost to warm the pool but as soon as the top was uncovered the heaters did nothing.
So he then started experimenting with oil burners to heat the water, at christmas he put the heater on at 10am and by noon there was steam rising from the pool and it cost him nothing to heat!