How does water circulate in the heater?

Basic physics apply: 

There are two holes inside the hot tub that operate as water inlet and outlet to and from the heater (below and above the LED light). Heated water flows upwards and through the upper hole (outlet) into the tub, while cold water stays nearer the bottom and keeps flowing into the heater through the lower hole (inlet). 

This cycle continues constantly while heating the stove. No mechanical pump is needed for this circulation.