I’m not able to find any estimates of the comet’s mass, so I’m kind of hard-pressed to offer concrete numbers depicting the aftermath of a hypothetical impact. Let’s just say it would be bad — very, very , very bad.

If we were to imagine a head-on collision, assume that the comet is mostly ice, and that it were to smack into the ocean, the impact would certainly cause incredible tsunamis and massive devastation (say goodbye to coastline cities). I remember reading that McNaught was traveling very quickly, so even if the comet’s nucleus was fairly small (say, one kilometer or so in diameter), the impact would release massive amounts of energy, a few hundred thousand megatons worth. That would pose huge problems for the overall climate in addition to wildlife, and displace a significant amount of water in the ocean — it wouldn’t leave it a dry valley as you put it, but it could certainly shift enough of a volume of water that it might (very, very slightly) affect the Earth’s rotation, much like the huge tsunami in December 2004. The death toll in human and other forms life would be unimaginably high. It would be an epic disaster.

A collision of that magnitude only occurs about once every 850,000 years though, so we can rest easy.