If the buzzer is a mechanical one then it will also generate a high voltage like the relay and blow up more transistors if the circuit doesn't have a protection diode..
Your diode is across the battery. The battery doesn't generate a high voltage, the mechanical buzzer does. Connect the diode across the buzzer.
The single transistor won't have enough sensitivity to detect water unless the water is very salty. Use the original two transistors and just replace the relay with the buzzer and include the diode across the buzzer if it is a mechanical one.
If the buzzer is a piezo beeper then the diode is not needed.
Then one or both of your transistors are either wired wrong (look at the picture of their pins on their datasheets) or they are burned out.
A transistor is turned on with base current. With nothing between the probes then the 1st transistor doesn't have base current and is turned off. Then the 2nd transistor is also off. Then the buzzer is also off.