Pyrete boats
I saw the MythBusters segment on making a boat out of frozen water and wood pulp, called Pyrete. It is essentially a composite material. Pyrete worked pretty good, except for it melting quickly. A boat might last in the Arctic winter for a while if the boat was made from pure water as the salt water there is a few degrees below 0°C. Even at best it would melt over the summer.
This material could be useful if the freezing point could be raised. Unfortunately, most things added to water lower the freezing point. NaCl, table salt, is the best known example. There are ways to raise the freezing point of water–certain organic acids, salts, sugar alcohols. But they require large quantities of additives, so they are either expensive or toxic or both. Too bad.