Images show the Boeing 737-800 on its belly and at an acute angle just above the water.
If it had stopped any further along the slope, the plane would have probably plunged into the Black Sea in the Turkish province of Trabzon.
The incident late on Saturday created panic among the 162 passengers and crew on board the Pegasus Airlines flight, but they were all evacuated safely.
Trabzon Governor Yucel Yavuz said on Sunday that investigators were trying to determine why the plane had left the runway.
The flight originated in the capital, Ankara.