An old fashioned 74LS00 needs an input current much higher than water can provide unless it is salty water.
The 74LS00 has a 4mA output current when low to light an LED dimly but its output high current is too low to light an LED (as you found).
To make the output drive an LED when it goes low then the anode of the LED must be connected to +5V and its current limiting resistor connects to the output of the gate. Connect both inputs to one probe and connect the other probe to +5V.
Make the surface area of each probe high by making them long. Space them close together in the water and add salt to make the water conductive.
The 74LS00 has a 4mA output current when low to light an LED dimly but its output high current is too low to light an LED (as you found).
To make the output drive an LED when it goes low then the anode of the LED must be connected to +5V and its current limiting resistor connects to the output of the gate. Connect both inputs to one probe and connect the other probe to +5V.
Make the surface area of each probe high by making them long. Space them close together in the water and add salt to make the water conductive.