Sailing some ships around the islands is the weakest exercise of sovereignty. Boots on the ground, folks, boots on the ground. That's what matters in territorial disputes. A lot of boots on the ground for a long time. If China wants to claim the island, it needs to land Chinese citizens on the islands, build houses for them, and keep them there for years. China would simultaneously need to prevent Japanese citizens from landing.
You must be wishing for war, then, because that's what would happen. Reguarly patrolling inside the 12nm "territorial waters" of DYT is a very significant move in and of itself, but certainly not one which would lead to war. Attempting to establish a long term presence on DYT would be interpreted by the rest of the world as a land grab since Japan is the de facto administrator of DYT, and would most certainly lead to war. Consider how China would take it if India sent some "civilians" into Aksai Chin and built some houses there and tried to defend them with its troops. You'd get a fight on your hands. It would be no different between China and Japan.
These islands are unihabited for a reson. There is nothing there to sustain life, and there is no reason...no real natural resources on the islamds themselves...to warrant settlements there. Everything would have to be shipped in to sustain them.
