Aquanaut's Holiday

Gameplay in Aquanaut's Holiday takes place in a first-person perspective and consists primarily of the player exploring vast stretches of ocean, occasionally discovering underwater ruins or treasure, or communicating with underwater creatures. The game has no time limits, enemies, or other obstacles. The game's objective, aside from exploring, is to build a large coral reef to attract a wide variety of fish and other marine wildlife.

Source: Wikipedia, "Aquanaut's Holiday", available under the CC-BY-SA License.