Supposedly, it was not the foam per se that caused them to get heavy. The concept was to completely fill the hulls with foam -- the kind you pump in and it expands and goes everywhere when exposed to air. The foam supported the shape of those hollow, stretch-formed aluminum hulls and made them very strong. And supposedly it was the kind of foam that has closed cells that cannot absorb water. The secondary idea was that with the hulls completely filled with foam, you did not even have to worry about getting water in the hulls and did not have to worry about draining them.
PROBLEM: The foam usually did not entirely fill the hulls in an even way, and there would be voids here and there, and you had no way of knowing where they were -- top, bottom, front, back..... So when water got into the hulls -- as water always does -- it would collect in the voids, and there was no way to get it out.
Personally, it seemed to me that the foam also absorbed water, even though it was not supposed to.

The Sizzler did not have a problem with stones puncturing it, as far as I know. It had a sharp edge all the way around each hull, fore and aft, where the two halves of the hull were sealed with a stainless steel "U" strip. It protected it well -- you could have pulled it up onto a concrete launching ramp without hurting the boat at all. But that sharp-edged bottom also was a problem for pulling it up on beaches, because it would dig in instead of sliding on the said. Definitely had to have beach wheels.