This visual novel has, probably, one of the best narratives in the medium. It takes a premise that sounds mundane (it's about a group of seven people trapped in a underwater theme park) and makes it interesting by adding a really strong cast of character that, in the surface, may seem like they are no more than anime tropes (the tsundere, the energetic girl, etc), but the way they interact and develop makes them really interesting and memorable. To top it all off, this game has, probably, one of the most awe-inspiring, unexpected, and daring endings of any work in the world of interactive fiction.
The sad part about this is the inaccessibility of this work: the original PC version of this game in English was released in 2005 so the program is really old and unstable and the translated script is not the best, so you have to download a fan-made patch to have a better experience. Recently, a new version was released on Steam, but it has the huge problem of being based on the Xbox 360 version/remake, which introduces changes to the original script of dubious quality (also, it seems that there are problems with the translated English script, again). It would be really nice to have an accessible way to play the original game, with the original script, on modern OSs.