http://en.wikipedia.org/wiki/Ice_nine (fictional material within Cat's Cradle)
This factoid gets trotted about without ever noting the (tiny) magnitude of the effect, it just plants a (false) idea in the readers mind that you can get ice faster by microwaving water first.
trying to work out why you couldn't use ice as a submarine hull. I mean the deeper you go the more freezes on it and the thicker it gets?
inb4 it floats, so does air
There were some plans to make ships out of it and it seems that mythbusters did an episode on it which I haven't seen: https://en.wikipedia.org/wiki/MythBusters_(2009_season)#Epis...
Thinking about it now, I wonder if a better form of pykrete could be made with a more modern expanding/insulating material than wood pulp…
As the article makes a point of mentioning, unlike nearly all materials, added pressure is more likely to melt ice than cause it to freeze, at least at the pressures and temperatures we're considering.
The pressure of water at 10000m (basically the deepest the oceans go) is ~100 MPa. Look at the liquid part of the phase diagram between 100 kPa and 100 MPa. See how it abnormally extends left as pressure increases to ~200 MPa?
If you keep it inside the vessel, then you're faced with containing a zone of steadily increasing heat (and thus steadily decreasing efficiency of your heat pump). If you try to dissipate it outside of the vessel, you run into further engineering problems (the heat conduit has to pass through the hull at some point, which creates a weak spot for pressure and a potential heat leak to the rest of your hull).