Water leaks from car when reversing

I suspect you're seeing the "condensate" from running the air conditioning system. That's the condensation that forms from the humidity in the air being cooled by the AC system. That water runs down into a plastic pan, then out of the vehicle through a tube. On most cars that tube has a rubber hose with a 90 degree bend in it hanging down from the passenger side of the firewall, under the hood. That dripping water is normal. In fact, it's removing that humidity that creates the most comfort from AC systems. Home dehumidifiers work the same way.

When you have a problem is when you don't see that dripping water. That's due to the drain tube being blocked by leaves or other debris. The symptoms include water dripping onto the front passenger side of the floor, and sometimes the sound of water splashing in the dash area. To fix that, the rubber hose is removed, then something like a pencil works well to poke into the tube to dislodge the debris.

What's probably happening is the water level hasn't gotten high enough to reach the drain port, but when you back up, the water sloshes into it and onto the ground, like it's supposed to.