The most common and most likely answer to this crossword clue is the 5 letter word NAVAL
I've seen this crossword clue in The Times
N
A
V
A
L
Buddy explains!

The straight part
We know that "of boats" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case of boats = naval.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 NAVAL mean?
NAVAL adjective- Of or relating to a navy.
- Of or relating to ships in general.