Pykrete was an experimental substance made during WW2 by a guy called Geoffery Pyke. He discovered that a mixture of 14% sawdust and 86% water, when frozen, made a substance lighter, but stronger than concrete, that floated and could take a lot of shots without too much impact, and melted much slower than ice.
There are 2 ways that it is said Pykrete was demonstrated, both equally plausable, one was that when Pyke was in a board meeting with Winston Churchill, that he, without a word, unveiled a block of ice, and a block of pykrete and shot them both. The ice instantly shattered, but the bullet that hit the Pykrete bounced off and hit the Defence minister in the leg. Instead of being outraged, he immediately set about developing Pykrete ships.
Another story says that Churchill was in the BATH when Pyke calmly walked in and set some pykrete in the tub with the Prime Minister. After waiting sufficient time and watching the pykrete resolutely not sinking or melting, he set about developing it.
Unfortunately for Pyke, he lost control of "Project Habakkuk"(Pykrete's codename) and it was taken over by the MOD. Subsequently, the war moved south into tropical regions, where the proposed "Ice-ships" wouldn't be feasable, so the only Ice-ship made at that time was the 100 by 20 metre prototype, which lasted the warm summer in a lake.