Here’s an overview of the game, via Bandai Namco:
A pirate’s life is fun, but dangerous. Go on a journey to the ocean together with the Straw Hat Pirates. The player is an apprentice pirate of the Straw Hat Pirates, and embark on a “one day voyage” on the Thousand Sunny. Adventures and events only unique to virtual reality will unfold, including navy shooting battles and communication events in which you’ll become closer with the Straw Hat Pirates.
And here are two screenshots:
One Piece: Grand Cruise will be playable at a Tokyo Tower event on July 22. Not only will attendees be able to play the game early, it will be a “special version that supports eight-player co-op play at once.”
A release date and price for One Piece: Grand Cruise have yet to be announced. The game has currently only been confirmed for release in Japan.
Update 07/17/17: View the above screenshots and artwork in high-res at the gallery.