Cuilionn, the earliest use of the word in print, found by Hugill dates to no earlier than the 1830's although reference to it's use is recorded by Felix Fabri in 1493. As to the orgin of the word, as Dick says is a crap shoot. Hugill puts forth 6 theories, breifly;
1 West Indians moving their shanties (huts) to song.
2 The drinking Shanties in the Gulf Ports where Black & White sailors, steveadores & hoosiers drank.
3 The boat songs of the old French voyageurs which were called 'chansons'.
4 The French word 'chantez' sing.
5 A derivation of the English word 'chant'.
6 A connection to the shantyboy songs of the lumber camps.
Hugill leans more to #5 then #1, with a lot of recent uncovering as to the extent the Afro American waterman has had on shanties & how far back their influence goes, I'd rather bet on #1, but then, it's only a hunch. So say what ever you'd like, no one can say you're wrong. Barry