The water salute is a touching airport tradition to honor military veterans, foreign dignitaries and new airline service. Salutes typically involve two firefighting rigs spraying arcs of water over an arriving or departing flight. It is a sign of respect, honour and gratitude. Each water cannon salute, which lasts about two minutes, can use as much as 3,000 gallons of water, so it does involve airport fire department resources.
A water salute consists on placing an even number of ARFF vehicles, namely crash tenders with powerful water cannons, on each side of a taxiway, pumping water to create a high arch. It can also be performed by three vehicles forming a triangle, with their water cannons’ stream meeting at the centre.Here are the main reasons for giving a water salute:
No one knows exactly when and where did water salutes in airports start. It is well known, though, that back to the days of the ocean liners it was common for fireboats and tugsto spray them with their water cannons when they were leaving or entering a port. So, water salute is not unique to airplanes alone and it is also given for ships as well. The idea must have come from there. Water salutes began being a common practice in the 1990s, when Salt Like City International Airport started saluting retiring Delta Air Lines pilots by creating a water arch beneath which the aircraft would pass.
Here is a video on how it looks from the inside of a cockpit of a Boeing 737 during a water salute.Source