The most common and most likely answer to this crossword clue is the 5 plus 3 letter word NOAHS ARK
I've seen this crossword clue in The Daily Telegraph
N
O
A
H
S
A
R
K
More clues leading to the result NOAHS ARK
- The Physician’s Gordon on an island rescue boat
- Transport ship in drama crossing area by Channel island
- Rescue vessel wrecked on a hot island
- Prototypal wildlife refuge
- Shark on with a cuckoo in floating zoo?
- Harks on about wrapping a vessel found in older book
- Expressions of disbelief and surprise, going by island in boat
- Ship from North America, with nothing on board, heading for Holy Island
- Shipping company that was floated using the double-entry system
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 = noahs ark.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.