Davy Jones is a monstrous and demonic being, variously shaped, in sailors' lore. The question that arises is whether he represents a genuine mythological being or whether he was just invented as a euphemism for drowning as Davy Jones's Locker means the place where drowned sailors go.
Davy Jones is not mentioned in earlier works. The first we hear of him is in a reference by Daniel Defoe in 1726. Did matelots of yore believe he was a real creature?
Some clue may be afforded us if we look at the origin of his name. Finding this poses a problem, however, as this origin cannot be determined. Some surmise it means "ghost of Jonah". Some would see in "Davy" the duppy of Jamaican belief. It has even been suggested that his name was suggested by an unfortunate short-sighted sailor known as Duffer Jones who, because of his affliction, kept falling overboard.
Lewis Spence has suggested, however, that he is some kind of Celtic spirit with a mythical pedigree. He points out that his name is possibly Welsh, Davy being a common Welsh name (St David is the patron of Wales) and Jones is widespread as a Welsh surname.
He also suggests that "Locker" may be a form of Ochren, which seems in Welsh legend to stand for the realm of the dead.
I like the Duffer Jones explanation myself.
Davy Jones was not always conceived in human form and may have been a shapeshifter. For example, see the picture below.