See, the water amount isn't that large. What if we crashed some comets that are mostly ice to Mars? Modern technology makes it easy to calculate some order of magnitude effects like what would be the average water coverage increase of Mars if Halley's comet (assuming it was completely water) was crashed there: 3 mm.

Calculation: https://www.wolframalpha.com/input?i=%28halley%27s+comet+vol...