It's a fair question. We don't let people flame people generally, and certainly not for asking an honest question.
I'm not the expert and I'm sure they'll kick in but here are what I see:
1. A hot water heater can deliver maybe 50-75 gallons of hot water, heated to about 125-140 degrees. I have no idea how many BTUs this represents but I highly doubt it's in the 85,000 to 100,000 range, which is what a heat pump generally delivers.
Heating a pool is all about BTU s or kilo-calories (metric version), not temperature. And you need a lot of BTU s.
Normal gas pool heaters run up to 400,000 BTU s because they are on-demand heaters and they can be expensive. I just don't see how a hot water heater, even an oil-fired one (about 3x as efficient as gas for the same gallons) is going to meet the demand needs.
2. The assumption that heat pumps are inefficient is based on a faulty premise. Heat pumps got their bad reputation by being used inappropriately to heat homes in climates too cold for them. When the air temp drops below, I dunno, 40 degrees, the H-P switches over to electrical resistance heating--very expensive and inefficient. But when the air temp is in the mid 40's to 50's. the H-P s worked reasonably well.
Now consider: When you go to heat your pool the air temp is going to be AT LEAST in the 50's (at night) and into the 70's or 80's or more during the day. You are now into the heat pump sweet spot. Now it can efficiently suck heat out of the air and warm your pool water with it. But it's not as on-demand as gas heating.
People with heat pumps generally find their energy bills for their pools are far less than those that heat with gas. Most people I know with gas heaters end up using them only very rarely--such as when the water is frigid and they have a party.
3. Solar is the cheapest of all--it's free. Al (Poconos) and I both use solar all summer and it keeps our pools comfortable. There are lots of solar solutions from home-grown (Al knows about them) to inexpensive, to unusual (like mine) to really outlandish and expensive.
4. Other people have replaced their A/C outdoor unit with water cooled ones that use the pool water. The A/C in the house works very well and the pool gets very warm. Other than the cost of the unit, like solar, this system is basically Zero operating cost.
Hope this all helps...
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