From The Atlantic (yeah, I know) a not-too-terribly scientific look at what a terraformed Mars might look like. Great pics:
The article ends by noting, "Looks like home, maybe a bit, just with a foreign geography." Except it doesn't, really. A terraformed Mars is going to have one giant super-continent. Even if you assume any bit of land that touches another bit of land constitute one continent (making North and South America a single continent, as well as Eurasia and Africa) Earth still has four distinct land masses. It also appears that a watery Mars is closer to a 50/50 split of surface area being water or land, as opposed to Earth's 70/30 split.
I can't imagine that doesn't result in some really odd weather. I do think the artist got it right that one half of that continent is really green (perhaps even waterlogged, like the Amazon) and the other half is pretty dry and barren; a watery Mars isn't necessarily going to be a green Mars. I could also see a wet Mars experiencing something like an epic version of our monsoon pattern weather.