Link to stream page: http://live.nicovideo.jp/watch/lv103480127
If you follow Miku’s page on Facebook, you would have seen the announcement about an upcoming water stage performance, and that’s essentially what this performance will be. The concert opens its doors on August 29th at 17:00 (5:00 PM) JST, and the concert starts at 19:00 (7:00 PM) JST.
Free users will be able to watch this event as well, but you will need a nicovideo account to watch it. If you do not have one, you can simply register an account at niconico.com, and use that account on nicovideo.
To avoid getting kicked from the stream, switching to the lower quality stream is recommended. You can do this by clicking the text below the player that says “低画質モード”. However, you should note that free users can still be kicked from the lower quality stream to make way for premium nicovideo users.
Have fun and enjoy the show!