I think the ice would be OK in the Vacuum of space.
Works for Comets.

The problem with trying to blast an Asteroid/Comet off a collision course has been two fold.

First the blast radius of an explosive deflection system is three dimensional so only the area against the target is effected, you loose bang out into space.

The Asteroid/Comet has a lot of delta V inertia stored up, objects in motion tend to stay in motion.

With Comets you have the conglomeration of ice and perhaps mountain sized chunks of rock and iron.

Blasting a Comet can get you a shotgun effect like a multiple war head impactor scenario.

I wanted to develop a lot of potential energy with a large mass and then get that wrap around effect of the semi liquid frozen slush transferring the deflecting energy without breaking up the impactor Asteroid/Comet.

The irony of a species that is composed mostly of water from a planet that has 2/3 of it's surface covered by water using a water based deflector hasn't escaped me.