Aqua Cube

Aqua Cube is a 2008 Puzzle Platformer doujin game for PC by Fox Eye, and one of their earliest titles in a healthy list of underwater-themed games.

It stars the Kobinata siblings: A young boy named Yuu, and his older sister Aya. These siblings love and care for each other so much that it somehow causes a rip in the time/space continuum during a beach trip, sending them both into a different dimension. Turns out this was the work of the evil Aquris, who rules over this new world and tried to pull one sibling from the other into this world, but ended up taking them both instead. So he curses Aya to be constantly captured and trapped inside giant cube-shaped water tanks that are found in this bizarre new dimension.

It's up to her little brother, Yuu, to save her from her watery prisons before she drowns, all while making sure he doesn't drown, himself. This will involves a lot of block-pushing, utilizing switches to raise and lower the water level, and avoiding contact with any and all dangerous obstacles he comes by. Later on, the brother and sister end up switching roles, turning Yuu into the hostage and Aya into the one that must rescue him.

Being one of Fox Eye's first games (and one Fox would love to eventually make a sequel to), this game set the standard for underwater-based gameplay and Oxygen Meter maintenance that Fox and his development staff would be known for in his games as the most prominent part of their challenge (among other things). This has shown well in his later games such as Holdover, Sacrifice Girl, and Hades Vanquish. The game's two-character management style eventually returned in Fox Eye's event-only title, Natica and Sandy: Underwater Rescue.

Both Japanese and English versions of the game can be purchased at DLsite.