The short answer is yes: putting water in your fuel tank will most certainly destroy, or at least severely damage your engine.
What will water in my fuel tank do to my engine?
If it has ingested the water already, it will refuse to start.
Water is not compressible, and if it enters the cylinders, it could cause the engine to seize, irreparably damaging it.
The presence of water will prevent combustion inside the cylinder, if the water content is high enough.
Also, water will cool the cylinder walls and cause non-uniform contraction and expansion, causing further damage.
One warning sign of water in your engine will be your car jolting or sputtering heavily when you try to accelerate, due to the inefficient combustion.
Also, water can corrode the fuel lines, injectors and pump, potentially resulting in thousands of dollars worth of repairs.
How can water enter the fuel system?
Firstly, water can enter through humidity, condensation or a mixture of both.
This is especially true if your fuel cap is not seated properly.
If your fuel tank isn’t sealed properly, rainwater could also enter the fuel tank via the filling mouth.
That is also true if you off-road frequently and get water up to the height of your fuel filler cap.
During the monsoon season, it’s good practice to check that your fuel cap is properly screwed in after filling up.
Thankfully, it isn’t the end of the world if water enters your fuel tank.
Have a mechanic check it out and if it’s serious enough, have the entire system drained and then refilled with fuel.
It’s expensive, but better than wrecking your engine.