Fish!

Fish! is a text adventure game by Magnetic Scrolls released in 1988.

An inter-dimensional terrorist group named the Seven Deadly Fins are threatening to take over the water-world of Hydropolis. It's up to a single Inter-Dimensional Espionage Agent to stop them—as a fish.

The game is a standard text adventure with static graphics in some versions. The player must warp to Hydropolis to foil the Seven Deadly Fins. But to do this, the three parts of the warp-device must be found and assembled. Each piece requires the completion of a sub-mission. The player's physical form - fish or human - changes as he progresses through the game.

Source: Wikipedia, "Fish!," available under the CC-BY-SA License.