Green’s Dictionary of Slang

(orig. RAF) to land one’s aircraft in the sea; of an aircraft, to land in the sea.

1946		G. Gibson Enemy Coast Ahead (1955) 175: He showed how to ‘ditch’ the aircraft, using slight flap and plenty of speed. The ‘drink’ we used for the occasion was a stretch of smooth cloud.
1954		N. Bartlett With Australians in Korea 133: The destroyer [...] had a pin-point position to steer for, where a Skyraider from the U.S. carrier Boxer had ‘ditched’.
1967		(con. 1940s) M. Dibner Admiral (1968) 433: I’m out of gas, fellows. I’m ditching.