Lots of questions I have on the boat but a few I ask here. On the starboard side there is a open close valve that comes from what looks like a holding tank, through the engine room and out the side. This is on a 38zx but the 33zx I looked at had the same. Last owner left it open and didn't know what it was for. AnyOne know what this is?

You're right, on my 33 that's the overboard discharge for the holding tank. Not sure if your's is the same but there is a key that you can turn tucked up in beside the rear seat that runs the pump.

I've looked everywhere on my 2008 38 ZR and have no idea how to pump it out.

I now know. There is a key in the cupboard in the head. It's hard to find in there but its there alright