took a pool noodle, bent into the letter "U" slid a thick contractor grade black trash bag over it, left the ends open, tied it to the side of the pool with a 3' string.
Water flows in and out of the open end, getting warmed up while trapped inside. Same concept as the DIY hula hoop lily pads...but with less work. made 2 of them in 2 minutes.
the string keeps it out of the intake suction....and I tied them on the side of the pool that gets the most sun.
You could stick your hand under the contraptions and feel a temp difference....its not going to heat the whole pool...but they don't hurt anything floating there and will add a little extra warmth when left in for days on end.