I have been thawing my uncooked chicken in my sink for years, and my family never got sick from eating it. Why can't you thaw uncooked chicken or any type of meat in your sink or on your countertop?
If you thaw raw meat at room temperature (> 5 celcius) you may get bacterial development. With chicken, not only can you get bacteria to develop, there is also a risk of propagating salmonella onto the surface which you are thawing the meat of and it's likely to contaminate other food too. I always recommend to let the meat thaw in the fridge, where the temperature is less likely to allow bacteria development, on a plate covered with plastic wrap, just make sure that no meat juice can espace and that the meat does not comes into contact with anything else than the plate itself and the plastic wrap, do this with all of your frozen meat.
Of course, wash your hands and working surfaces after working with raw meat.
Agree with maximegir above (point given), but just in case you're thinking, well, that's okay, when I cook the chicken any bacteria present will be killed, there's another factor to consider. Bacteria will have been present in the chicken before it was frozen, all living things have bacteria; once it's frozen, the bacteria are arrested by the freezing process. Defrosting in a refrigerator obviously means it's cold enough still for most bacteria to be unable to function normally, but if you defrost at room temperature, parts of the chicken will be warm enough for bacteria to start doing their thing. When bacteria are active, they produce by products which, in some cases, are toxic to us, and these toxins are not inactivated or destroyed by the cooking process. So, in some cases, illness might be caused by eating chicken defrosted at room temperature and then cooked, even if its cooked properly.
You might be surprised to hear this, but studies have shown that there are more bacteria in the average household kitchen sink than in your toilet! Due to this extremely high amount of bacteria, this bacteria can infect your meat products.
As a rule of thumb - if it falls in the sink, throw it away!
On the other hand, if you first put the meat in a sealed plastic bag, then it should be safe to thaw in water, as long as no water enters the bag. The bag will shield the meat from bacterial exposure.