The most common and most likely answer to this crossword clue is the 9 letter word SEA-GOING
I've seen this crossword clue in The Guardian
S
E
A
-
G
O
I
N
G
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 = sea-going.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 SEA-GOING mean?
SEA-GOING adjective- Travelling out to sea.
- Made for, or used on the high seas.
- Fit for sailing on the high seas.