Inflatable pools come in many sizes and shapes. Some are small enough so that the sun's heat warms the pool water to a comfortable level. Larger inflatable pools may need more warming options than just the sun.
Install the pool in a sunny area. The best and least expensive pool warmer is the sun. Installing in the sunniest, flat area will provide day-long warming.
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Cover the pool every night and when not in use with a solar cover. This will not only help warm the water when the sun is out but will also help retain the water's heat at night.
Install a solar heater. If the water in your inflatable pool does not warm up enough to make swimming enjoyable, consider purchasing and installing a solar heater. These are relatively inexpensive and can raise the pool water's temperature about 10 degrees.