Sounds like you would need a hot water heater that you can control the heat levels on and pipe it out to the outdoor pool. You may want to check with a regular pool service as they would be familiar with heating swimming pools.

Hi
I've done it in an above ground 1000 gallon with two standard 350watt submersible aquarium heaters During cold spells i wrapped the outside in styrofoam and kept bubble wrap on the surface . Be sure to have some circulation to spread the heat around.
Why do you need heat?? Tropical fish?? Most plants can handle down to frost. gary

Thanks for the replies. I have guppies to eat the mosquito larvae. 99% of the year the water is perfect. There's always a few chilly days in South Florida though.

why keep the water at 70?....do you have tropicals?

as long as a small hole is kept open to allow for gasses to vent from the water, the fish will be fine in frigid water. I have a couple of "heaters" that run on 25 watts. they look like floating bird baths and allow water to fill them and keep the ice open there. most floating heaters run on 1200 or 1500 watts.....

There will be no ice accumulation in south Florida :-) Your guppies will be fine with no heater. It's takes a while for the water temp to fall even when the air is cool. Maybe just a thermal blanket to pull over the top if prolonged cold is forecasted.