Almost all at sea? (2,3,4)
I believe the answer is:
in the main
'at sea?' is the definition.
('the main' is an old name for the sea)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for in the main that I've seen before include "Primarily" , "All things considered" , "Where those who walked the plank finished" , "where to find water" , "Where cod and haddock may be found" .)