This swimming system is a new feature, and is broken in a gameplay sense. Notice that the people saying that players dying by swimming 1 tile are stupid are the ones to whom it hasn't happened to yet. To those of you, just wait until you lag into the lake, and lose a month of character building and progress, and see how funny you find it then.
That aside, I'm confused by some of the new terrain types. It appears as though there are 3 types of swamp; the leechy marshes we are familiar with, a brightly colored marsh with nothing in it, including no plant life or decor (those are the bits that look like snow on the local map), and a darkly-colored swamp with little rivulets running through it. There are also 3 kinds of Plains; pink colored grasslands, normal green grasslands, and then yellowed grasslands that are devoid of anything.
I suspect that terrain is broken into 3 categories per type, one with a decidedly "Pretty" or "Good" appearance, a Neutral/Natural appearance, and then a "Dark" or "Evil" appearance. Whether this will lead to different Herbs and animals in each, I don't know.