The swan song is a metaphorical phrase for a final gesture, effort, or performance given just before death or retirement. The phrase refers to an ancient belief that swans sing a beautiful song in the moment just before death, having been silent (or alternatively, not so musical) during most of their lifetime. This belief, whose basis in actuality is long-debated, had become proverbial in Ancient Greece by the 3rd century BC, and was reiterated many times in later Western poetry and art.
The "Mark of Charon" is an insect, probably a honey bee. In ancient Greece, burying someone with a coin (usually placed in the mouth, but nowadays depicted as on the eyes) paid their passage on Charon's ferry. These were generally obols, a low denomination coin; some were even cheaper, stamped foil shaped like a coin. Each region or city used an identifying coinage image; Athens had an owl, Ephesus, in eastern Greece, used the honey bee.
The term "swan song" has become an idiom referring to a final appearance. In most TV shows with an episode of this title, there is usually the death of a main or beloved character.
When Hook is in the Enchanted Forest his hair is parted to the right. When he is in Storybrooke, as the Dark One, his hair is parted to the left.
Charon's boat first appears empty and surrounded by a rectangular base. Later it is full of people and this base is not visible. Cinematic prop boats are often not watertight and only float because they are bouyed up by floatation platforms. These platforms are calibrated to be underwater at full operational load.