Masato listed the chords from the Leadbelly songbook, but remember that Leadbelly tuned his guitar down about two tones from concert pitch. While the songbook shows Titanic in the key of G, Leadbelly, though singing in G, was actually playing in a C fingering.

It was [C]midnight on the sea[C7]-
Band playing, [F]"Nearer My God to Thee"[C]-
Cryin' fare thee, Titanic, fare thee well[G7]-
It was [C]midnight on the sea[C7]-
Band playing, [F]"Nearer My God to Thee"[C]-
Cryin' fare thee, Ti-[G7]tanic, fare thee well[C]-.

Also, his C chord has the pinky on the third fret(G)of the high E string, and then slides up to the fifth fret (A) on midnight. On the G chords hes uses a similar pinky movement to make the chord alternate between G and G7.

Pete Seeger had an instruction record, "12 string guitar as played by Leadbelly" about 50 years ago. It would help if you can find a copy.
FAB