I hear a vague resemblance between DS and RTOCA. It may simply mean that the use of one (presumably DS) unconsciously influenced some singer(s) to adopt the other for shipboard use. Or it may be completely coincidental.

Years and years ago I noticed an Irish reel that I thought was surprisingly reminiscent of DS, but I don't remember the name and can't say whether the resemblance would seem as striking today.

It was probably in O'Neill's collection (1903), so even if it's quite similar it could have been influenced by the shanty rather than the other way around. Most frustrating.

FWIW, my view is that DS counts as a "real" (if perhaps unusual)shanty because not only was it used as one and thought of as one by various shanty collectors, it seems not to have been sung in non-shanty contexts.

It's minor point, however.