If you have more water in the fuel than you think you do (there always is some due to condensation in the tanks) then you might be able to reach your destination but you'll be burning more 'fuel' than your original estimate would have you believe because there is less power per unit weight of (contaminated) fuel.
This is fairly common in GA and there are cases where it has happened in scheduled flights as well. That's why fuel sampling is common practice.
Interesting. That makes sense. Thank you for the explanation!
It's supposed to be an extremely low amount and the fuel pick-ups are placed such that it should never be a problem but there have been cases where water in the fuel caused problems, including at least one notorious crash where the cause was identified to be fuel contamination.