Inside each bottle there is a 3.5 meteres piece of 4 mm drip watering tubing. This heater takes advantage of the greenhouse effect inside the bottles.
It is like the heat inside a car in the sun in summer. Since when the circuit is stopped the water heats more inside the tubing it woul be a good idea to use a timer with i. e. 1 minute ON and 1 minute OFF.
This also would protect the pump motor from excessive heating. With a small water pump the water is cycled and the temperature rises. It would be better to use a 12 volt DC motor water pump connected to several solar cells.
You can also try to paste a sheet of aluminum foil in order to concentrate more heat inside the bottles.
The solar water heater is scalable, so you can connect more bottles and groups of bottles in order to heat a swimming pool.