According to the OED:
b. slang. (a) trans. To bring (an aircraft) down into the sea in an emergency. (b) intr. To come down into the sea in an emergency. Cf. ditch n.1 2c
1941 Times (Weekly ed.) 15 Oct. 19 The pilot..must ‘ditch’ his aircraft in the sea, near enough to a ship for him to be picked up.
The reference to DITCH points to:
c. (a) Naval slang. The sea; (b) R.A.F. slang. The English Channel or the North Sea.
1922 Man. Seamanship I. i. 33 A smart seaman would not talk officially of the sea by a favourite slang expression ‘the ditch’.
1925 E. Fraser & J. Gibbons Soldier & Sailor Words 78 ‘He fell into the ditch’, i.e. overboard.
1945 E. Partridge Dict. R.A.F. Slang 23 The Ditch, the sea; especially the English Channel.
The ditch reference goes to even earlier slang for any waterway. So I suspect Louis's citation is correct.