Abstract: An improved heater for a spa bath is disclosed. The heater includes a body having an inlet and an outlet connectable to the pipe work of a bath for housing a heater element for heating water passing through the heater. The disclosed heater also includes an optical water sensor housed in the body for detecting the presence or absence of water in the heater body. The optical water sensor preferably includes a light source, a transmitting medium for transmitting light from the light source, and a light detector for detecting the light as transmitted through the transmitting medium whereby the transmission or internal reflection characteristics of the transmitting medium are affected by the contact of water to an external surface of the transmitting medium. A spa bath including the aforesaid heater is also described.