The most common and most likely answer to this crossword clue is the 9 letter word SALTWATER
S
A
L
T
W
A
T
E
R
Buddy explains!

The straight part
We know that "marine" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case marine = saltwater.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does SALTWATER mean?
SALTWATER noun- Any water containing dissolved salt; brine
- New Keynesian or Keynesian, in reference to macroeconomics and economics departments on the East Coast and West Coast of the United States of America.