Placing a buoy in Japanese waters isn't assertive, but aggressive.
> The open-sea area in question is surrounded by Japan's EEZ.
Japan has no special rights outside its EEZ at all. Within the EEZ, it has rights to resources below the surface, but a buoy on the surface would be allowed. The territorial waters where a country can limit most surface activity are much smaller.
>Last July, China installed another buoy inside Japan's exclusive economic zone
as well as
>with Japan protesting against repeated intrusions by Chinese ships into Japanese waters around the Senkakus