You can use the same pin_b3 that was used for the LED (output of low coolant indicator) instead for the buzzer. Use resistor, NPN transistor PN2222, and buzzer as shown in this schematic: https://www.google.com/search?q=schematic+sound+buzzer+2222&tbm=isch&source=iu&ictx=1&fir=ukD_xMA9HYGZOM%253A%252CQFrrs-dO1GKZxM%252C_&vet=1&usg=AI4_-kS17xBTyTEiungClrFz6l_wRjfksA&sa=X&ved=2ahUKEwjS5Ym50-biAhUqxVkKHQCtCZQQ9QEwAHoECAAQBA#imgrc=ukD_xMA9HYGZOM:You will definitely want to use probes that do not corrode in water. Don't know what the current draw was.
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