I presume the piece you are holding is what you broke off from the hole where the wooden bung is inserted.
As an old mechanic
I would taste the water to determine sea or fresh but that's just me. Looking at where the bung is I am guessing it is fresh water from the engine and you can let it drain out.
You need to determine if the bit broken in the hole where the bung is is really gone and the threads are clear. If you are lucky it is. If you are not lucky you need to extract the broken bit without damaging the threads. If you aren't skilled you may want to get a mechanic
. the procedure is to get a screw extractor (also called "easy out") - lube it up with penetrating fluid (WD40) and then try to extract.
Looking at the bit in your hand it seems there is a body of some type and the left end in the photo
is simply plugged. Unless I am looking at it wrong and something was connected there.
If it is just a plug
you can take the bit in your hand to a plumbing
supply store and find just a plug
that has the same threads as the bit that went in the hole.
With the hole clean and a suitable threaded plug in hand you simply apply some sealant
(RTV) to the plug and screw it back in. Service
the fresh water system and you are done.