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My guess is the foam you've seen that was sodden was polyurethane. It's well known that this breaks down over time and becomes swater-logged.
EPS and especially XPS (extruded) styrofoam does not.
Hence why I suggested XPS foam, as it is the better oif the two with the smallest and tightest 'air bubbles' and least voids for any water penetration.
The bottom line is, would you rather be afloat after hitting a container, or awash?
The problem with the bilges is that often you can't get to where the damaged area is, as it's usually inside a sealed buoyancy chamber, or completely covered by gear and crap, and by the time you can get to it, the hull is awash, eg Ppalu.
If you hit something offshore, you need to be able to float in order to sail or motor to shore and get hauled out. There's bugger all you can do to repair a badly hole dhull at sea, short of old fashioned 'fothering', although doing so with modern materials like caulking and plastic sheeting is more effective than tar-smeared canvas.
You will need to haul out regardless of where the damage is.
On lead mines, with full length bilges, with a removable sole, having access makes sense, but multis are constructed differently.....making the 'desirability' of an accesible bilge less important than overall floatation ability once holed.
Lead mines go to the bottom, regardless of where they are holed, but a small hole, or a leak, might be able to be stopped or plugged, if you can get at it, hence the need for accessible bilges.
If the foam is contigous, epoxied to the bulkheads and to itself, it forms a solid lump of foam, effectively a giant 'airbag' at the bottom of the hull, where that floatation effect is most needed.
. And glued in place with silicone.
