As far as what EBarnacle said earlier on (oh, oh, thread creep) about timming & putting yourself in an area where the odds of hearing something different increases, I very much have to go along with except that the odds get slimer daily as the old timers or those that knew the old timers die off. That would leave us with only collecting what already's been written or recorded. Though some of what's been written or collected may have been seen & not heard & some songs heard & not seen, so there's another area. I believe the days of finding unknown sea shanties sung by sailors that worked under sail is "nearly" gone. Maybe in the areas of the West Indies or around the Ackerland (SP?) Islands where the deep water trades outlasted most other areas (up till WWII). As to finding some of the missing shanties among the West Indies it's likely to be next to nil if not worst with only titles & no content to compare with. Some other ares that are possiblies are other traditions that may have some link. An example would be the work song "Drinking that Wine" which is found as a net hauling shanty & as a prison worksong (axe/logging) & another (IMHO) would be "Won't You Help Me To Raise'Em" found in the same fisheries as "Drinking that Wine" & found in the southern prison system as a varient of "O,Alabama".
In the late 70's I was given the title of shantyman on the museum brig, Carthaginian (Maui). I'd help in my spare time lending a hand & a voice in her restoration. An old timer, George Herbert, a master rigger from West Geelong who had worked under sail in both the Baltic & Cape Horn trades dropped by to lend a hand to some old friends of his but more to show & advise them. He loved to sing & play his concertina, uke & mouth harp. At a singing party held on board I ended up taping the whole evening & not being educated in the fine arts of collecting I was happy just to have sung with him. After playing these tapes for 15 yrs I wrote to him & up till he died we swapped tapes of sea songs (me still not finding out much about the songs except that I could only find some not so close versions in print). I never found out where he got some of these (I can make some good guesses though) but my point here is that for me it was pure blind, dumb luck (not the only time my luck has been this good to me) & with serious luck someone else may have uncovered much more at the time, the other side is that to find any old timers these days would take more than serious, blind, dumb luck. Sorry, I'll give up on my thread creep ramblings & you & me a rest, happy hunting
Barry