Tap water contains both small amounts of fluoride and sodium. "Soft" water contains 20 mg or less sodium per litre, which is the best for you. Altogether, you drink about 30-50mg of sodium from water per day.
Tap water is better for you than bottled water, it contains fluoride that you can't taste, that cleans your teeth, if you've gone to the dentist a few times before, they put different flavours of fluoride in your mouth. Also, once water has been sitting in a plastic bottle for too long, harmful chemicals in the plastic start to leak into the water. Look at the bottom of your plastic containers before you store food or water. You'll see a triangle with a number in it, 1 is the best while 10 is horrible. Most bottles are in the 6-8 range which is still very bad.