The trickiness of life
The mermaids are told that if they can get a human husband, their souls will become immortal. But what they're not told is that if they exert energy to do that against the wills of men, that they almost always kill them—the are literally sirens after all. Only one mermaid finds her enlightenment in her non-violent strategy of permanent isolation on a desolate island.
The cruel circle of fate
Just like the Greeks at war, Sirena must learn the hard way that life has seasons, and even though life is kind enough to give us what we want on certain occasions, the understanding should always be that everything on earth is temporary—even mortal love. She learns this the hard way when her beloved sails away to fight in battle, leaving Sirena to contemplate her newfound, profoundly lonely immortality.
The quest for immortality
The story is about death in that the mermaids kill sailors out of their own fear of death, and since fate ultimately calls the hero Philoctetes to return to his inevitable death march to Troy. In one way, the book is about showing that immortality may not be the ultimate virtue in the universe, especially if it's a lonely immortality.
