Intense in the sea (4)
I believe the answer is:
deep
I believe this clue is a double definition.
'intense' is the first definition.
(associated in meaning)
'the sea' is the second definition.
(thesaurus)
'in' is the link.
(Other definitions for deep that I've seen before include "dangerous for swimming?" , "profoundly" , "hard to follow" , "Mysterious" , "greatly involved?" .)