I second what FF said.
That valve looks more like galvanised or plated to me, NOT stainless! And, it almost certainly will have tapered threads.
You can't fix it properly just by cleaning it up and using some kind of sealant.
You really need to remove, inspect, and probably replace with a proper, UL approved marine seacock, like a Groco (there are others). Note that there is also no backing block!
Every boat I have looked at in the past two years, except one, had this same poor set-up. So, first on my list of things to do with almost any boat I purchase will probably be through hull / seacock replacement!