The most common and most likely answer to this crossword clue is the 8 letter word CHOPPIER
C
H
O
P
P
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R
Buddy explains!
The straight part
We know that "when sea gets rougher" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case when sea gets rougher = choppier.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 CHOPPIER mean?
CHOPPIER adjective- (of the surface of water) Having many small, rough waves.
- Discontinuous, intermittent.
- (of wind) Shifting, variable