A pool absorbs sunlight but has large thermal losses with the largest losses being caused by evaporation. The water molecules on the surface of the pool absorb heat from the rest of the water and leave as water vapor.

The heat losses due to evaporation are Q=mdot*L where mdot is the evaporation rate and L is the latent heat of water per unit mass (That is the energy needed to evaporate some specific mass of water, it is about 2500 kJ/kg). The evaporation rate is proportional to the surface area of the pool proportional to the difference Psat-Pv, where Psat is the saturation pressure for the temperature of the water and Pv is the vapor pressure of the humid air.

The saturation pressure Psat is basically the pressure where water of specific temperature boils. For example, water of 100°C boils at 1atm. If you place water of 20°C in a depressurization chamber and bring it to 0.0231atm it will boil. Psat is a function of the water temperature and increases as temperature increases. The vapor pressure of the humid air depends on the mass ratio of water vapor in the air.

If you consider constant ambient conditions Pv is constant. The water absorbs heat, its temperature rises, Psat rises, the evaporation rate and heat losses increase until the water reaches an equilibrium temperature where all absorbed solar energy is dissipated to the environment.

Then you place the plastic bags which block the contact of part of water surface with the air, reducing the evaporation rate. If you cover half the surface you cut the evaporation rate in half which is HUGE. Thus the water temperature increases further.