The most common and most likely answer to this crossword clue is the 8 letter word TRIREMES
I've seen this crossword clue in The Daily Telegraph
T
R
I
R
E
M
E
S
More clues leading to the result TRIREMES
Buddy explains!
The straight part
We know that "boats" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case boats = triremes.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 TRIREMES mean?
TRIREMES noun- (history) A galley with three banks of oars, one above the other, used mainly as a warship.