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No matter how badly you want water and air, there are limits. You certainly don't want a hundred billion tons of anything crashing into your planet in one lump. That would cloud the skies with more dust than even Mars has seen for a very long time, not to mention shaking up everything around.
So you have to take some precautions. Before your approaching comet gets that far, you site demolition charges in its core, carefully placed to shatter it into the tiniest pieces you can manage. (The pieces won't be all that tiny, anyway, but still.) Most of the fragments will burn up or at least volatilize from air friction—even Mars's thin air is enough to do that—and the seismic impacts of the residues as they strike will be, you hope, tolerable.
You don't want them crashing into the surface at an excessively high velocity, either. So you navigate your comet to come up on the planet from behind, so that both are going in the same orbital direction around the Sun and the combined speeds are minimized. Then, at that last moment, you fire the deceleration jets from the Augenstein drives you have forethoughtfully already installed in the comet in order to slow it still more—so that your impact velocity isn't much more than one or two kilometers a second. Then you jettison the Augensteins, cross your fingers, and hope.