Utterly lost (3,2,3)
'utterly lost' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for all at sea that I've seen before include "Utterly confused, totally on the ocean" , "Where the entire fleet in wartime might be" , "very lost" , "Rescuers at Dunkirk were thus" , "muddled?" .)