Cork lifevests were known to cause injuries for some time before their "type acceptance" was revoked in the U.S. A WW-II manual for merchant sailors ("How To Abandon Ship," Richards & Banigan, Cornel Maritime Press) advised on pages 20 and 21:
"Do not jump into the water....If you are wearing a lifejacket containing cork, you are in danger of breaking a rib or your collar bone."
The main advantage to cork was incompressability which meant the floation was not degraded in stowage. Kapok, a natural air-trapping fiber, replaced cork, but can be compressed by weight or tightly-packed stowage. This reduces the available flotation. Kapok also becomes waterlogged while cork is relatively immune to this problem.
Kapok eventually won the flotation battle in part because it allows better design of lifevests. The flotation can be concentrated on the wearer's chest and behind the head. This floats the person with mouth and nose in a better position for breathing, even if the wearer is not fully conscious. And, someone wearing a kapok lifevest is less likely to suffer broken bones (shoulder as well as neck) when falling or jumping into the water from height.
Like cork, kapok is no longer "type accepted" due to its potential loss of floation in storage and when wet. Today, accepted lifevests must have closed-cell foam as the flotation medium.
Titanic's cork life vests were white canvas. White remained an acceptable color through WW-II when international orange became the required hue. White vests are highly visible against dark water, but may be confused with white foam or even small bits of ice when searching for victims of a shipwreck. Nothing except distress equipment (rafts, lifeboats, lifevests, etc.) on the surface of the sea is international orange.
-- David G. Brown
