Drain valve will not tighten

Brand new boat owner. I have a 2006 Bayliner bowrider 225 with a 4.3 Mercrusier engine. I picked it up from the repair shop after it was winterized and noticed antifreeze was coming out the drain plug in the back of the boat. I smelled antifreeze. I know that the water is drained out of the block into the bilge through a blue valve handle. so I tried to tighten it to see if the mechanic forgot to close the valve but the valve will not tighten and the handle just continues to turn clockwise and counter clockwise. There is a long metal stem that goes from the valve handle down to I'm assuming the valve. If I pull up on the handle it moves up about a quarter inch and I can press it back down. Either way it will not tighten.

Any thoughts as to why the valve stem is not closing the valve and the antifreeze continues to drain out? How difficult is it to replace the valve and or valve stem? What parts would I need to replace them?

I appreciate any help. Thanks

you have the single point drain and when the plastic water housing breaks and it will remove that quick crack system and install manual drain fittings

After accidentally slipping and dropping my foot into the engine bay and breaking off the #$%!! plastic valve, thereby dumping all the non enviro friendly antifreeze out of my FWC engine's block into my bilge, I replaced the whole fitting with a brass plug.

the single point system only drain the raw water side

On mine, the FWC was an aftermarket kit so the broken blue plug drained the block.