The most common and most likely answer to this crossword clue is the 8 letter word BOATYARD
I've seen this crossword clue in The Sunday Telegraph
B
O
A
T
Y
A
R
D
Buddy explains!

The straight part
We know that "Seaside facility" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case seaside facility = boatyard.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 BOATYARD mean?
BOATYARD noun- A place where boats are built and repaired.
- Shipyard