The previous example didn't have that problem because crew weight is also supported by the water direclly below the bottom and the end decks need not support any crew. I've seen several fiberglass boats that used solid foam slabs to effect a solid double bottom. That had large openings in the transoms to drain any water. In truth these boats are really foam boards with fiberglass streamlined bump shields. But watch out.
Foam weighs more than people realize in the volumes needed.
If the crew hops onto the sideways boat she will sink down less that six inches provided the side boxes are at least that deep. And when righted there will not be a drop aboard. She's ready to go. This setup is often seen on side decked daysailers that were common in years past. Those boats usually didn't really have boxes along the side decks but if water doesn't slop over the edge of the decks the effect is the same.