As time goes on, game companies appear to be more focused on better graphics than better gameplay. This has led to 3D graphics and fewer innovations in almost every new game released. Aquaria is one of the pleasant exceptions to this rule. It is both rendered in gorgeous multi-layered 2D and quite innovative in how it handles progression and story.
Aquaria's gameplay is very simple, but it has surprising depth. For example, movement is purely 2D, but you can ride currents, grab walls and jump off them for a quick boost, turn into a special form to ignore currents, switch to a small form and swim faster, and create blocks to interrupt the current using a plant form. All of these are used at some point in the game, and most are needed to battle the numerous bosses of Aquaria.
Your unlockable forms aren't limited to just movement changes, though. One shines through the occasional dark area, one creates different varieties of plants, one allows you to swallow fish whole, one shoots blasts of energy, and one allows you to fit in tight spaces. There are ample opportunities to use each power, and there are many small puzzles that require you to use a clever combination of one or more of them. Aquaria is never boring.
In order to activate all of these powers, you need to sing a specific combination of three or more notes once they are unlocked. If you ever forget the combination, you can go to the powers screen and get a quick reminder. They're quite easy to remember, though, so you won't find yourself doing it too often.
That's not your only reason to sing. You'll find plants that release a treasure if you sing their note, bosses that require a Simon-says style song to take damage, and unlockable zones that require a hidden song. These songs do not show up in the reminder window, so it's up to you to learn them. In this respect, Aquaria is a little like Myst. You never know when you'll need a song you saw on the wall elsewhere, so you tend to write down anything you see.
When you finally sing your way to the truth behind the state of the underwater world, you won't need to stop there. There are dozens of collectables spread throughout the world, and the majority of them require significant effort to find or to get to. You can play for hours just enjoying the world and searching for another item to add to your collection. Overall, there's nothing turbulent in the waters of Aquaria.