The most likely scenario in my view is one where the Hong Kong vessel plays a cat-and-mouse with the Japanese Coast Guard around the territorial limits of the Islands, then goes back unharmed but without actually making a landing. This is unfortunate for the Chinese authorities because it gives the Japanese authorities a chance to effectively exercise sovereign powers over the Islands. Under another, less likely scenario, the Hong Kong vessel makes an improbable landing, whereupon the vessel and its proprietors are immediately apprehended—we assume that they really are political activists and not Chinese fishermen, who will not hesitate to kill if need be—transported to the Naha Coast Guard office as per the usual catch-and-release program for simple illegal entry violators. This is unfortunate for the Chinese authorities because it also gives the Japanese authorities a chance to effectively exercise sovereign powers over the Islands.
A problem arises when there is an incident involving a physical altercation. We assume that they really are political activists and not Chinese fishermen, who will not hesitate to kill if need be, but accidents happen. It’s simple when only the Hong Kong vessel or its occupants are harmed, because the result is merely a variant of one of the first two scenarios, depending on whether or not they actually make landfall. It is when harm befalls the Japanese Coast Guard, its ship or personnel, that things become interesting. In the September 2010 incident involving a Chinese fishing boat, the Kan administration wound up losing significant political face when it delivered the captain and the boat to the Chinese side before the public prosecutors pressed charges. But this time, the Japanese authorities will be up against, if I’m not mistaken, the Hong Kong Government, an administration with priorities and leverage that are very much unlike those of the Beijing Government. The Noda administration will find it much easier to face down its opponent than the Kan administration did two years ago. Again, this is also unfortunate for the Chinese authorities because it also gives the Japanese authorities a chance to effectively exercise sovereign powers over the Islands.
I think that it boils down to this: If you have possession, you don’t want to give the other side reason to contest it (think South Korea and its Dokto); if you don’t, then you don’t want to give the other side the opportunity to exercise sovereign powers. I think that the Chinese authorities miscalculated.
ADD 2.0: It was the Okinawa Police (nice touch), most surely riding on the Coast Guard vessels, who made the arrests. And in retrospect, it made much more sense to let them land, then make the arrest, since that was much safer than trying to repel the Hong Kong vessel offshore. In any case, the protest only reinforced the sense of Japanese control over the Islands. When you think about it, that’s almost always the case. Sea Shepherd, Anonymous, they all reinforce the notion of sovereignty even as they challenge it. If I were the Chinese authorities, I would certainly discourage these private protests against Japan’s exercise of its sovereign powers over these islands.