Nuclear weapons are very expensive, even to the Chinese Government. They are also very very heavy - unless they are a tactical nuclear device, which has a small explosion radius compared to traditional nuclear arms.
The Chinese Government isn't going to just float one of them across the sea and hope that it hits a good target, as it could easily get taken by unexpected weather and potentially drop in the Pacific.
If they WERE to not care about the costs of a tactical nuclear device and float one anyway - blowing it up over unpopulated Montana is still the best option.Q