Tell us about yourself!
Just doing the math here ... you'll have a more efficient system if continuously pumping. Efficiency goes up with a higher delta in the heating element (tube). For example ... with a heating element running at 210F getting water from 200F to 210F will take a lot longer than from 100F to 110F.
absolutely, which is why they make black covers, but who wants to pull that on and off all the time, and pools look better when you can see the water. Pool covers essentially say: "neener neener, can't swim"
Also ... probably loosing a lot of heat due to convection. Make edging with 2X4's then throw 2 plexiglass sheets on top, then put pipe insulation (cheap) on the pipe going to the water. Those whose systems seem really inefficient are probably in cold, perhaps breezy areas.