Updated: May 26, 2020
For those who don't know but are curious, we are bringing the answer to this question. There is no exact date or location when this event started to be a common practice in the aviation industry, but it is obvious that this tradition came from the tradition of celebration of the new sea routes. In the 1990s, Salt Lake City was the first airport which marked the retirement of Delta Airways captain in this way, so it looks like that this tradition became normal in the aviation industry from there.
Water salute is a ceremonial event in the aviation industry when aircraft get sprayed over by water by two firefighting trucks. Two trucks are standing opposite of each other of each side of the stand and at the moment when aircraft enter the stand, two truck starts to spray the water above the aircraft by creating the high arch.
The water salute is organized for a few reasons:
First or last flight for the particular airline in the particular airport,
First or last flight of specific aircraft,
The first flight of specific aircraft type,
Captains or air traffics controller retirement.
Here is a video of water salute to Air Berlin on its last flight which was held on October 27th 2017., the company went bankrupt so the Berlin Tegel airport prepared this huge ceremony to say goodbye to this airline. Approx. from 6min at the video, you can see how that looked in this specific case.
Water salute is a clear mark of showing respect, honor, and gratitude in the airline industry and all of those who work in it know how emotional and special this event is. Of course, the event is interesting for passengers as well who can see what is happening from the terminal building. At the aircraft, the captain will announce this event upon landing, so passengers can understand what is really going on.
Event is quite popular in our airports in Croatia as well, especially at the start of the summer season when airlines start with operating new routes.