Flotation should be sized and located so as to keep the boat floating and stable when swamped.
To tie in with what Jack said, it will not make the boat more difficult to swamp.
If you add, say, five pounds of foam to the boat, and locate it as high as possible in the hull, then you just lost five pounds of carrying capacity, but if the boat ever gets swamped, the buoyant material will help to prevent it from sinking, and depending on how well you were able to locate it, will help keep the boat stable while you bail it out.
Are we making sense?
GBinGA