Keep in mind that juice and sweetened drinks also provide a lot of extra calories (most of which are sugar which I have been studying all the adverse effects of lately), so don't rely on them too much.
Also try to keep your caffeinated drinks to a minimum and refer to my post on caffeine for more information on that.
Why is drinking water so important? Water facilitates the absorption of essential nutrients into the cells and transports vitamins, trace elements, minerals, and hormones to the blood cells. It's those nutrient-rich blood cells that reach the placenta and ultimately your baby. And don't forget, you're not just eating and drinking for two, you're also expelling waste for two. Water dissolves the waste products and helps flush them from your body.
Don't cut down on your water and other fluid intake because you're afraid of retaining water, either. Oddly enough, the more fluids you drink during pregnancy, the less your body retains. So if your feet and ankles are swollen, drinking more water actually helps decrease that. Drinking water also helps regulate your body's internal cooling system which all pregnant ladies know is necessary (I am currently in a thin house dress and still too warm). Plus plenty of water can help with exhaustion and headaches so feel free to develop a drinking problem.
Another thing to note is an interesting discovery I made. Not only is it important to space your water intake throughout the day (because it is possible to drink too much at once, flood your system, and feel painfully bloated), but it also matters what vehicle you use to transport the water to your mouth. I like to drink from water bottles. I have several of them which I refill as needed. They are great for keeping cat hair from drifting into my beverage, or dust particles, or anything else that makes me dump my cup and refill it. More importantly though, I was using them as a way to track how much I drank throughout the day.