How bad for your health are sodas? And what's a healthy or at least sort of okay replacement for soda? I already drink lots of water.

None. There is no soda on the market that is healthy. None of them have any nutritional value - they are either sugar water or water with chemicals acting as artificial sweeteners. They also contain salt and acids which in high quantities and years of consumption can lead to high blood pressure, kidney disease, and teeth ; bone erosion. The best replacement is water followed by limited amts of juice.
Try tea. Iced tea, hot tea, lemon tea are all reasonable choices for a soda substitute. A fresca every now and then is also a reasonable choice. You can mix the soda with juice, water or tea to make a dilute drink.
Tea, seltzer. The problem with soda is the large amount of sugar. If you drink a lot of regular soda on a daily basis, you are most likely consuming a lot of empty calories, and will gain weight. In my opinion, a moderate amount of diet sodas do no harm. I usually recommend hot or iced tea, unsweetened. Try to avoid juices, as they too, are high in sugar.