Science vs. superstition
The emerging role of science in society (the telescope Mary uses, and the discussion of astronomy) is contrasted with folklore and superstition (references to the tooth charmer, ghosts, legends).
Rural exodus into urban centres
Many young people were forced to leave their rural homes to find work in the big city. Salt-Water Moon deals with this rural exodus (Jacob leaves Newfoundland for Toronto).
World War I and Newfoundland
Newfoundland made a significant contribution to World War I, and experienced many losses during the Battle of Beaumont-Hamel.
The play examines traditional gender roles. Jacob doesn't announce his departure to Mary, but assumes they will just pick up where they left off when he returns; Mary is initially passive about Jacob's betrayal (she misses him, but doesn't actively seek to contact him while he is in Toronto). She is also willing to marry Jerome out of necessity more than love.
Heroism and courage
Heroism and courage recur throughout the play, in Mary's desire to rescue her sister, the references to Beaumont-Hamel, Jacob's feelings about his father rocking the cradle on William McKenzie's porch, and Mary's visit with Dot to see Tommy Ricketts, for example.
Social class divides
There are many examples of social class divides in Salt-Water Moon: The Dawes, who employ Mary; William McKenzie and his treatment of Jacob's father; Jacob's desire to change social class by leaving for Toronto; Mary's desire to change social class by marrying a young man of stature.