Davy Jones' Locker is a phrase meant to refer to the bottom of the sea; the resting place of drowned seamen.
Etymology
Davy Jones is the supposed evil spirit of the sea. There are various possible sources of the phrase. Jones may be a corruption of name of the biblical seaman Jonah who spent 3 days in the belly of a big fish (possibly a whale). Alternatively, Jones may have been a real character - a 16th century publican who owned a locker much feared by sailors. Lastly, DJ may have taken his surname from Jonah but first name from Saint David, the patron saint of Wales and guardian of Welsh sailors.