Factual error: After the plane is submerged, Jack Lemmon says, "... this plane is pressurized!" Jet aircraft are pressurized by bleeding air from the compressor sections of the engines into the cabin; in other words, the plane is only pressurized while the engines are running. Also, aircraft are only pressurized to a few psi above the outside air pressure at altitude, and never to more than the air pressure at sea level. The water pressure would be a great deal higher than the air pressure in the cabin, and since aircraft are not water tight, the water would quickly fill the aircraft. (If the aircraft was in 50 feet of water, the water pressure would be 21.7 psi, versus a maximum cabin pressure of 14.7 psi.)
Continuity mistake: In order to show the take-off of Stevens Corporation's 747, a scene from "Airport '75" is used - the plane's tail fin reads "CA" (for "Columbia Airlines") instead of showing the Stevens Corporate Logo.
Factual error: In the many shots of the plane underwater, both from outside and looking out from within, the "sea" resembles the bottom of a swimming pool more than the bottom of a shallow area of ocean, as not a single fish or any other aquatic life (which would be abundant in areas around The Bahamas) is ever seen.
Factual error: The entire premise of a jumbo jet descending into water and remaining grossly intact, much less air/watertight, is so flawed on its face as to be laughable. In the handful of real life controlled jetliner ditchings and/or controlled flights into water ("CFIT/CFIW") accidents, in every case the bottom of the hull has been shredded and the airplane took on water instantly and sank within minutes. Those were the "good" water landings. In anything less than that, the wings and engines would be ripped off and the fuselage fragmented as well. The exception that proves the rule is, of course, Sully Sullenberger's landing on the Hudson, in which the hull, wings, and engines remained intact.
Factual error: In the opening credits, the plane is showing landing over the Capitol Building on the way to Dulles. Dulles is no where near the Capitol Building.