With plenty of sun--get some black drainage pipe, the corrugated 3" stuff.  Stretch it out in the sun so the entrance is higher than where you want it to exit.  I get about 10-15 degrees higher water temperature at 25 feet.  Helped us get in the pool a couple weeks earlier and stayed in it for awhile longer.  I found this on the web some time ago, the next step is to build a plastic box and use a coil of the pipe, may increase temp by another several degrees--I haven't figured it was worthwhile yet.