My take on Kipling's verses is that it's a whaler about to go to sea again and musing on the fact that his business makes it impossible for him ever to see his wife thru a pregnancy. I would expect the continuation (if any) to deal further with his & his wife's feelings, rather than to further stages in the voyage.

As to whether Kipling made up the verses, I dare say he might have; but he might also have heard them in the field. He was living in Vermont and spent a good deal of time in Gloucester.