As per TW
Close when leaving the boat for some time.
'Some Time' I dunno, maybe a week or so?
Leave 'Open' whilst the boat is 'active'
One thing I do with My Cock is to manipulate it from time to time.
The Sea cock too.
They tend to seize up if they are not 'worked' regularly.
Made of steel and bronze/ brass + the salt water element, tiss a recipe for disaster if not moved now and again.