The most common and most likely answer to this crossword clue is the 9 letter word WHALEBOAT
I've seen this crossword clue in The Sunday Telegraph
W
H
A
L
E
B
O
A
T
More clues leading to the result WHALEBOAT
Buddy explains!

The straight part
We know that "seagoing vessel" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case seagoing vessel = whaleboat.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 WHALEBOAT mean?
WHALEBOAT noun- A long narrow rowing boat, formerly used in whaling, which is pointed at both ends so that it can move either forwards or backwards equally well.
- A boat resembling this and carried on a warship or other ship.