How Can I Heat My Pool?

Covering the pool with a black cover will help it heat up a bit while also reducing the water's exposure to sunlight, and so reduce algal growth.  A two-layer or inflatable cover will do an even better job of holding the heat in.

A submersible water heat can help but make no mistake, there's a LOT of water in even a small pool, and the wattage required to heat that up even a little bit will blow you away.  A pool holding just 250 gallons contains about 2000 pounds of water; raising that much water's temperature just ONE degree requires running a 1000-w heater for about half an hour.

And that's just 250 gallons.  Your pool is probably larger, and you probably want more than just one degree of heating.  The demands get pretty heavy pretty fast.

Cheaper: about 300' of black garden hose coiled on the roof of your house.  Plumb it up so the pool's pump pushes water through it.  The faster it pushes, the more efficient it becomes, don't worry that the water doesn't feel hot; you're trying to make it less cold.

Turn the pump off when the sun goes down, or close valves so water doesn't go through the solar coils.

Look this up online, many folks have done this, it's super cheap and effective, the two best features you could possibly ask for.  Combined with a two-layer cover this can make for a significantly extended swimming season.  Add insulation around the outside, that'll help a lot as well.

The best thing to do is get a smaller pool, like a jacuzzi tub and hook it up to your hot water heater.   Put the tub outside and drain the plumbing on the few days it freezes.   By having it outside, you don't have the mess.  It is sort of funny, if it looks like spa it can go outdoors.  If it looks like a bath tub, it has to go indoor.  You can be odd and put a bath tub outdoor.

If you want the water temp to be above the ambient temp, you are going to need a huge solar system or a mechanical way to heat it.  Plus you are going to need to slow down the heat loss, foam around the walls and solar blankets and such. Especially to retain the heat during the night. The size of the solar heating system you need depends on the size of the pool. For myself, In New England during the summer. With insulated pool walls, solar heater, solar covers. I  am only able to get the pool temp up to about 5 degree's of ambient. I can't retain enough heat during the night to make up for it during the day from mid aug on.

other than passive solar, there is no low cost way to heat that much water -- there are 8+ gallons per cubic foot.  And all passive solar systems, afaik, require significant upfront investment -- grampa

Solar heating will be the cheapest option with maybe another form of heating as top up. You should also use a floating blanket overnight to trap heat.