As a native of the southeast, we learned this as a child's song, but it is my understanding (through word-of-mouth in Gullah circles) it originated as an underground railroad song. The runaway slaves made up all sorts of little songs with homonym words used as directions for safe passage, an oral roadmap. "Rock Island Line" (a mighty fine line) referred to a rail line used by the slaves, as did "C-Line", which became Sea Lion (the Woman part may have referred to a woman who assisted the runaways board the C-Line in the underground railroad). Many whites as well as blacks offered assistance. Perhaps the woman sat at a particular place and drank tea or coffee as a signal to those needing help... in spy-like fashion.
"Wade in the Water" is another Und.RR song...if you wade, n footprints to follow and it confuses tracking dogs.