It serves as something of a predecessor to the aquarium introduced in the sequel, which plays a larger story role and is generally more fleshed out.
"Mankind's creativity enhances the splendors of nature in a microcosm all can enjoy."
How to Unlock
After taking at least one dive with a dolphin partner and adding at least ten creatures to the Marine Encyclopedia, Katherine will tell you about the aquarium's existence, and an icon will appear to the west of your map. Clicking this icon after selecting the steering wheel in the Gabbiano's cabin will let the player enter the aquarium at any time. Visiting the aquarium is largely optional, but in order to progress through the story, the player must dive here at least once, thus unlocking the Underwater Camera and the Great Aqua Cave.
After the player has completely filled in the encyclopedia, namely by discovering every creature and unlocking all three tiers of trivia for each one, then they will be granted the opportunity to place the Ancient Mother inside the tank as well.
The Aquarium is presumably located on the mainland of Manoa Lai Island, which is located to the east of Manoa Lai Sea, although the map depicts it as being to the west of the Great Drop-Off. Not much is known about its appearance (save for a couple of establishing shots angled from outside the tank), nor anyone who works there, but it consists of a single tank in which any non-zoom-mode creatures can be placed. The player is restricted in terms of what they can do inside the tank; they cannot make use of the Underwater Whistle or Pen, nor feed the animals they've placed.
Upon arrival, the player will automatically be submerged into the tank, and is unable to explore any other parts of the aquarium. Once inside, they will be given the choice to place any creature they've unlocked, either by selecting them manually or by choosing the "autoplacement" option. The tank is arranged in a 4x4 grid format, and each creature will either occupy one, two, or four squares on it, thus limiting the amount that can be placed inside. Size notwithstanding, there are no restrictions on what creature combinations can be placed in the aquarium; even the two legendary creatures in the game can be placed here, and the player can even add multiple specimens despite only one of each being viewable in the wild.
Oddly enough, the player also has the option to enter the aquarium at night, unlike in the second game. This is likely due to the fact that the entire game takes place in one location and time-zone, as opposed to multiple, like in the sequel. It is also possible that the player's position as the curator might give them the privilege to enter after dark.