sea water aka salt water needs to be washed off.
if it was drenched,
I would remove the batteries buy several gallons of drinking water & several bags of
100% all natural Hardwood Lump Charcoal & flush the detector down,
inside & out.
then let it set for at least a week, maybe a month.
in an enclosed area on top of the ripped open bags of Charcoal
Example
Once I was absolutely sure it was Dry completely,
I'd put batteries in & try it.
If it still doesn't work I'd send it in for repair
Of course if $$$ are no problem, Just send it in now with a note.
Please be advised, this is what I would do.
I'm not saying I know what I'm doing
of course if it was on & submerged, this is a waste of time