Ok, technically water released into space would likely boil, not vaporize of freeze.
(EDIT: Of course, since vaporizing IS boiling, I guess I was off a bit here. 
) The reason for this is that water freezes by being reduced in temperature, and to do this it has to radiate its heat out. Vacuum is a very poor conductor of radiant heat. Also the vacuum of space is low pressure by definition and the boiling point of water lowers as the pressure drops.
That being said you could supercool the water before releasing it, or add some chemicals to it that would keep the boiling point artificially high I suppose. You could also keep it pressurized in some sort of membrane while you waited for it to freeze. This would actually make a lot of sense as it would allow you to shape the ice shields the way you wanted.
My biggest question about this is why would ice be a shield? It sounds to me like this would be another kind of armor.
But if you want the ice to be considered shields, the ice would be partially effective at blocking projectiles and missles I would think. Maybe you should make the standard weapons blocked by shields.
Also, to your question. An ice plate over the hull would interfere with boarding I would think. They would have to cut through it to get to the docking ports, or to the hull itself.
Geoschmo
[ August 26, 2002, 14:21: Message edited by: geoschmo ]