The most common and most likely answer to this crossword clue is the 7 letter word WETSUIT
I've seen this crossword clue in Financial Times
W
E
T
S
U
I
T
More clues leading to the result WETSUIT
Buddy explains!

The straight part
We know that "At sea, use it in worst extremes?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case at sea, use it in worst extremes? = wetsuit.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 WETSUIT mean?
WETSUIT noun- A close fitting, insulating garment usually made from neoprene or similar material designed to keep one warm during activities on or under water.