The Aquatic Planet is a world full of water, which is one of many planet types and, one of 5 extreme planets in Galactic Civilizations III.
How to Identify?
Aquatic worlds can be identified by the water icon next to the planet and when hovering your mouse over the planet, it will indicate it's an aquatic world. If the planet is not able to be colonized it will give a message in the description that indicates "Requires Extreme Planet".
Aquatic worlds are mostly water or swamp land, which makes it ideal for aquatic life. Unfortunately, for most races this makes it a planet that does not easily sustain life forms. Because of this it is one of five extreme planets which require additional technologies or racial abilities to be able to survive in this environment.
One may use the Adaptable racial ability to gain an early advantage and the ability to colonize this planet type, especially in a galaxy crowded with races and limited expansion opportunities. Suggestion to colonize normal planets first and extreme planets second. When choosing which Aquatic planets to colonize choose something on the border of your neighbors ZOC (zone of control [influence]) to attempt to persuade inhabitants in the area to your influence.
Which races able to immediately colonize?
Currently there are two races in Galactic Civilizations III that are able to colonize aquatic planets at the start of the game; The Yor Singularity, and any custom faction with the Adaptable race trait.