Pykrete was a secret weapon. The British intended to build really gigantic aircraft carriers of the stuff. Just make your hull thick enough, and any torpedo damage will have no effect. If your ship is made of super-ice rather than steel, you could build mile-long floating islands. For a permanent ship, add some hoses used as refrigeration coils to keep it from melting at all. Unfortunately the war ended before any major use was made, and today such a large slow target so far from populated areas just crys out for a small nuke bomb. Only one small test-ship was ever built. See http://www.combinedops.com/Pykrete.htm

I always wanted to make some Pykrete to play with. Unfortunately there's a problem. It takes forever to melt, but it takes just as long to freeze! Mr. Pyke apparently intended to make Pykrete by injecting the water slurry through cryo-nozzles which would supercool the mixture. The liquid would be far below 0C, and when sprayed into molds it would harden immediately. Making a block of Pykerete is not a simple proposition! I never had the ambition or the weeks of freezing time to mess with the idea.

However, when discussing this on The Straight Dope and Metafilter forums I noticed a neat trick. (I haven't tested it, so I don't know if it will work.) Don't supercool anything. Instead take a cue from nature: big piles of snow. Big piles of snow melt very slowly. Compressed snow is a thermal insulator: like styrofoam made of ice. If you wanted to make some artifical "compressed snow" by freezing some water-foam, it would take forever to harden. The outer layers would insulate the still-liquid interior of your ice block, and freezing would halt.

So instead, just make some snow and then pound it down into a compressed block. It's part water, part air. Styrofoam made of ice. THAT'S IT! The same thing should apply to Pykrete. Just add wood dust.

Make a piston and cylinder out of a pipe and a hunk of aluminum. Make some snow (or finely shaved ice), and mix it thoroughly with the correct amount of very fine sawdust (supposedly 14%.) Put it in the pipe and use the piston to hammer it down to form a solid block. Instant Pykrete! You could even try some simple supercooling: take your Pykrete block down below 0C degrees in the freezer, then soak it with icewater to glue the snow together and fill in some of the air gaps.

Practical joke: while it's snowing, blow some wood flour continuously onto the growing snow layer. (Hmmm, does wheat flour work? Easier to find some.) As the layer piles up, it will form Pykrete. That pile of "snow" might still be in your yard in August!

Impractical non-joke: mix snow and sawdust during below-0 conditions and dump it into the ocean. Repeat. Repeat. A huge floating snow-pile builds up. The weight of overlying layers compresses it into a solid. This is how glaciers are born, but glaciers can shatter, and glaciers can melt in warm weather. After you've made a few hundred million tons of the stuff, climb on board and declare your floating island to have national sovereignty. What will you name it? Habakkukia?

How about "Pykistan!"      :)