The Eastport pram design flotation is sealed voids under forward and aft seats, but the volume is too small to provide much buoyancy. To be successfully bailed out when swamped, a boat has to maintain a little freeboard with somebody aboard. There isn't much room under the seat to add more there.
Not an issue for ordinary use. But, say, for a child to be able to self-rescue by bailing, you need real positive flotation--hence all that foam.