I agree with Chris, fit a thin strip of wood glued round the hull just below deck level for the deck(s) to sit on. For the straight sections you might be able to use 6mm square. Something more flexible or a series of blocks aroumd the curved bow section. It is also a good idea to fit cross beams as well in places where they don't restrict access to the interior as they will support the deck across the width of the model. All this will help strengthen the hull.
Whichever glue you use, as Ray says, roughen up the surface of the styrene hull to provide a mechanical bond.
As a belt and braces suggestion, once the deck has been glued in place, run a little Deluxe Materieal Superphatic glue around the joints. This is very thin and will 'wick' into any cracks. Although usually used for timber to timber joints it also provides a good bond betrween porous and non porous surfaces including wood to plastic where it is surprisingly effective . I use it all the time as a little goes a long way.
**LINK**
Colin