Pykrete, the Super-Secret, Super-Weapon of WWII

See this interesting article on
Pykrete, an ice blend, strengthened tremendously by mixing in wood pulp as it freezes.

...you can mold pykrete into blocks from the simplest materials and then plane it, just like wood. And it has tremendous crush resistance: a one-inch column of the stuff will support an automobile. Moreover, it takes much longer to melt than pure ice.

What would we have done with it in WWII?

Pyke envisioned ships as vast and solid as icebergs. You could make the sides of your boat tens of feet thick, hundreds if you felt like it, and bullets or torpedoes would bounce away or knock off pathetically ineffectual chunks. And when a torpedo did knock a chunk away—so? You were floating in a sea of raw repair material. Given how long it took pykrete to melt, and the minimal onboard refrigeration equipment needed to stay frozen and afloat, it would be months or years before the boats exhausted their usefulness. In battle, the ice ships could put their onboard refrigeration systems to good use by spraying super-cooled water at enemy ships, icing their hatches shut, clogging their guns, and freezing hapless sailors to death.