The most common and most likely answer to this crossword clue is the 8 plus 6 letter word FLOATING VOTERS
I've seen this crossword clue in The Daily Telegraph
F
L
O
A
T
I
N
G
V
O
T
E
R
S
More clues leading to the result FLOATING VOTERS
Buddy explains!
The straight part
We know that "Politicians’ targets all at sea?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case politicians’ targets all at sea? = floating voters.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.