What is the sea **** for?? Engine cooling
(supply/discharge)? Sea water for the sink? For Wash down water? Sewage overboard? Or Toilet flushing system?
Now can you live with out it? Need It?
Can you plug it from the outside and try to see if you can repair
it by removing the old valve, chase the threads and put on a new valve?
Do you have to replace the entire through hull fitting?
But what you have described is a boat sinker
if left unattended. It must be taken care of, immediately. In the mean time have a couple of plugs ready for usage.
Check all of your other through hull fittings also. To see if there are others that may need to be replace.
It could be a one or two day haul out if below the water line
. Or see if a list on the boat will bring it clear of the water and repair
it from a punt. But a haul out would be the best way to do it.
Oh!! Make sure your pumps
can keep up with that size hole if the valve breaks off. And have an alarm system on you pumps
so you know when they start up.