Why is water coming from the drainage point on the underside of the car after
a long Journey?
You have not really said were or what type of water is leaking. However possible reason is do you have the aircon on cold. If so when you stop the engine the aircon also stops working the warmth of the engine warms the condenser which has been very cold so drips water. All depends on length of time on your journey and how cold. It is nothing really to worry about just shows the condenser is working correctly.