During Annan Water, William makes a deal with the river to allow him to pass just once so that he can be with his love one final night. He knows he's already made a deal with his mother that he could only have the one night before she reclaimed him (a la Wanting Comes in Waves/Repaid "And if I grant you this favor, to hand you your life for the evening I will retake by morning"), so rather than be ripped from his true love once more, he chooses to be swallowed in the Annan waters.
Meanwhile, as he's crossing the river, The Rake is haunted (or driven mad if you like) by the ghosts of his murdered children. They drive him out (as I'd like to imagine, before he rapes Margaret) freeing William to rescue his true love. She agrees to follow William to their death in the Annan rather than be torn apart again, so one final time he heaves the makeshift raft into the water to carry them off.
As they drift the river begins to claim its prize, and William asks Margaret to marry him before they're taken. The chorus, where they sing "But I pulled you and I called you here (Didn't I? Didn't ? Didn't I?)" is sung by both Margaret and William, because in a way both have rescued the other. Margaret first helps William who was wounded while in the form of a faun, and later calls him to her during Margaret in Captivity. She also in a greater sense frees him from his mother, who wouldn't allow him to leave the Taiga. William frees Margaret from the Rake, and calls her to him in Won't Want for Love.
“And these hazards of love never more will trouble us”. Margaret and William now in death are free.
So, here's my interpretation of the song:
During Annan Water, William makes a deal with the river to allow him to pass just once so that he can be with his love one final night. He knows he's already made a deal with his mother that he could only have the one night before she reclaimed him (a la Wanting Comes in Waves/Repaid "And if I grant you this favor, to hand you your life for the evening I will retake by morning"), so rather than be ripped from his true love once more, he chooses to be swallowed in the Annan waters.
Meanwhile, as he's crossing the river, The Rake is haunted (or driven mad if you like) by the ghosts of his murdered children. They drive him out (as I'd like to imagine, before he rapes Margaret) freeing William to rescue his true love. She agrees to follow William to their death in the Annan rather than be torn apart again, so one final time he heaves the makeshift raft into the water to carry them off.
As they drift the river begins to claim its prize, and William asks Margaret to marry him before they're taken. The chorus, where they sing "But I pulled you and I called you here (Didn't I? Didn't ? Didn't I?)" is sung by both Margaret and William, because in a way both have rescued the other. Margaret first helps William who was wounded while in the form of a faun, and later calls him to her during Margaret in Captivity. She also in a greater sense frees him from his mother, who wouldn't allow him to leave the Taiga. William frees Margaret from the Rake, and calls her to him in Won't Want for Love.
“And these hazards of love never more will trouble us”. Margaret and William now in death are free.