Sea, informally styled (6)
I believe the answer is:
themed
I believe this clue is a double definition.
'sea informally' is the first definition.
(the Med can informally mean the Mediterranean)
'styled' is the second definition.
(styled with a particular theme)
(Other definitions for themed that I've seen before include "with thread?" , "given a conceptual treatment" , "Given recurrent idea" , "Like special crossword" , "setting" .)