Occasionally I see an item that I feel is innovative and a bit outside of the box. Recently I found this little device – Beluga Solar Pool Heater. Now I am not sure how well it will work, so let me just make a disclaimer that I am not endorsing this product. It is an interesting concept though and quite simple.
The basic premise here is that you screw this into a threaded 1-1/2″ return fitting. These fittings are standard on almost all vinyl liner pools, but not too common with gunite pools. Once the device is attached to your pool return, it diverts a small amount of water through an attached garden hose, recommended to be black rubber, and then you route this hose back to the pool. The water from the hose ends up being hot from laying out in the sun.
Sure there are other ways to accomplish this, but this device is ready to go if you are willing to spend the money on it. Most cheap heating solutions are gimmicky and this one is no exception. Depending on your application, this might be just what you need to avoid a cold pool next summer.