The most common and most likely answer to this crossword clue is the 8 letter word TOADFISH
I've seen this crossword clue in The Guardian
T
O
A
D
F
I
S
H
Buddy explains!

The straight part
We know that "Marine creature" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case marine creature = toadfish.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 TOADFISH mean?
TOADFISH noun- Any fish thought to resemble a toad