Don't use edevices when driving. Smart watch included. It might not be as bad as texting on a phone, would definitely be worse texting on a watch.
Can you imagine someone trying to fiddle with their dial on the side of the watch, while holding the other hand in a position on the wheel that allows them to see the screen. Talk about an accident waiting to happen. Even if you don't touch the dial, and only look at it passively, you are still engaging in a process where you must remove your had from the wheel to twist your wrist to see the screen(unless you put the watch on upside down, then you could twist your wrist without removing your hand. Then take your eyes off the road long enough to read your text/twitterfeed/whatever.
This has nothing to do with apple, its true of all smart watches, or any other device. Its distracted driving to interact with them, no matter how you try to justify that one thing is less distracting then another. Its still very wrong to use these devices while driving. People are dieing every day because of people who cant put down their ecrap when they are driving. Really, you'll survive until you get to your destination, you don't have to check your twitter/facebook/email every 5 seconds while driving.
No matter how good you think you are you are NOT, it only takes a few tenths of a second to kill someone, and thats hwo long you will be looking at your edevice minimum. Turn the self centered portion of your brain off while driving, and just think about the families you will potentially destroy because you couldnt keep your eyes on the road.