Usually at sea? (2,3,4)
I believe the answer is:
in the main
I believe this is a double definition.
'usually' is the first definition.
(I've seen this before)
'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 "On the whole" , "Where ships are" , "Swimming" , "mostly" , "Where those who walked the plank finished" .)