And, after further delving into the subject, I found some evidence that some Carley floats were made with balsa. Scroll down to the third left hand side page in this link:
http://www.defence.gov.au/sydneyii/Parliamentary%20Submissions/PINQ/SUBS/010/PINQ.SUBS.010.0049.pdf
It would seem that it depended on the manufacturer of the float as to what material they were made of. There was a war on.
From good ol' Wikipedia:
Also, in 1943 the US developed a balsa wood liferaft that would not sink, irrespective of the number of holes (from enemy fire) in it. These balsa liferafts were designed to hold five to ten men on a platform suspended on the inside or fifteen to twenty-five hanging lines placed on the outsides. They were inexpensive, and during the war thousands were stored in any space possible on US warships and merchant ships. These liferafts were intended only for use during a short term before lifeboats or another ship in the convoy or group could bring them aboard. When the USS Indianapolis, a cruiser operating alone, was sunk in 1945, none of its larger lifeboats were launched, and the survivors had to rely on balsa liferafts automatically released as the ship sank; many of the crew perished, but the balsa liferafts saved others; ultimately 316 of 1,196 crew survived
So, I would say that US made Carley floats were, more than likely, built with balsa wood. You were probably correct in the first place, Garth.
Lee