The deck is not flat and the hull is built using two straight "runners" where the superstructure will be, the ribs are held in place by the runners until the hull is finished and then the runners are cut off to the deck level and give the deck something to attach to around the superstructure opening. I then added 4 strips of 1/32" ply to the inside of the runners and up to a level about 6mm (1/4") above the deck to provide a water runoff barrier or an "upstand". The Superstructure then fits snuggly over this rectangular ply upstand. The superstructure in my model is built as one piece but if you want to segment it that would be fine, my suggestion would be to split it in front of the day cabin and add some more water barriers where it's split. I have the bridge floor at the same level above the deck as the section aft of the day cabin which seems to have been true of the real boats but it's not 100% clear, it would make sense to do that to stop water sloshing about in the bridge area!
