Mostly at sea? (2,3,4)
I believe the answer is:
in the main
This clue is a double definition.
'mostly' is the first definition.
(both can mean generally)
'at sea?' is the second definition.
('the main' is an old name for the sea)
(Other definitions for in the main that I've seen before include "For the large part" , "All things considered" , "Where schools are found" , "Swimming" , "Where piped water collects" .)