The most common and most likely answer to this crossword clue is the 3 letter word HOY
I've seen this crossword clue in The Guardian
H
O
Y
Buddy explains!

The straight part
We know that "boat" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case boat = hoy.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 HOY mean?
HOY noun- A small coaster vessel, usually sloop-rigged, used in conveying passengers and goods, or as a tender to larger vessels in port.
- To incite; to drive onward.
- Ho!, hallo!, stop!
- To throw.