Thank you so much for your important question.
Indeed the practice of watering the grave is among the recommended acts. The tradition says: "It's among the prophetic tradition to sprinkle water on a grave while facing the direction of qibla, starting from the spot of the head to the feet" Sheikh Tusi in book al-tahzib, volume 1, page 320.
Another tradition reported from Imam Jafar Al-Sadiq says: " it reduces the punishment or stops it" Some scholars say: "It's more recommended during the period of 40 days after one's departure but there is no harm in doing it now and then when one visit his or her loved ones"
In conclusion, the practice is true
Sheikh Nuru Mohammed