I just built my first galleon - a pretty thing indeed! But how do I build anything on it?

I see here an earlier question about galleon platform size, but I don't seem to find the "platform" at all - whatever I try to place on the galleon (items from the "advanced rafts" mod, vanilla items, vanilla foundations, s+ foundations) I only get a "no ground or snap point" message. I tried bow and stern - anywhere on my galleon - to no avail.

Also, am I supposed to get inside the galleon somehow? I was briefly there when placing it in water, but there seems to be no return below deck...