I’d got into mishaps at sea halfway into vessel (9)
I believe the answer is:
amidships
'halfway into vessel' is the definition.
'i'd got into mishaps at sea' is the wordplay.
'got into' indicates putting letters inside.
'at sea' indicates anagramming the letters.
'mishaps' anagrammed gives 'amships'.
'id' put into 'amships' is 'AMIDSHIPS'.
(Other definitions for amidships that I've seen before include "At the centre of a boat" , "moving towards the driest place on board?" , "Centrally on board" , "nautical location?" , "core on board" .)