Short video of how I hooked up the GAME solar water heaters to a solar powered water pump. With this flow rate the water coming out is 4 to 5 degrees warmer than the water going in.
It was early June and the outside temps were probably 85 and the pool does seem to be 2 or 3 degrees warmer than this time last year (they have been hooked up for over a month now). If you decrease the flow rate the water will get actually very warm but as far as thermal transfer to the pool you probably wouldn’t gain anything.
These things get bad reviews on Amazon and I understand why. If you used a hi flow high pressure pump as suggested in the instructions I doubt there would be any heat transfer at all. But with this little solar water pump the pressure and flow are pretty low and I think its allowing them to work as advertized.
If you were to use a similar setup at a cabin to heat up maybe 40 gallons (continuous circulation) for hot water I have no doubt it would work great.