The most common and most likely answer to this crossword clue is the 6 letter word DESALT
I've seen this crossword clue in The Times
D
E
S
A
L
T
More clues leading to the result DESALT
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The straight part
We know that "something to do with seawater?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case something to do with seawater? = desalt.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 DESALT mean?
DESALT verb- To remove salt from; to desalinate.