The most common and most likely answer to this crossword clue is the 8 plus 5 letter word ATLANTIC OCEAN
I've seen this crossword clue in Financial Times
A
T
L
A
N
T
I
C
O
C
E
A
N
More clues leading to the result ATLANTIC OCEAN
Buddy explains!
The straight part
We know that "deep water" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case deep water = atlantic ocean.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.