Two Tides is split into two halves - a dangerous surface region with live enemies, and a maze-like depths that will tax air.
Near the surface, there will be plenty of sharks and jelly fish as hazards, and one vortex drone to the far right. They mostly serve as obstacles to the depths at best, but are easily evaded. If desired, there is a Pulsar found in the middle-left cave to attack in multiple directions at once.
To proceed, head to the bottom-right tunnel to the right of the lower sunken ship. This tunnel has rock hazards that need to be dodged, but will end in a key glyph (1). Sing at the glyph to obtain the key. The route forward has a strong current, thus you will need to backtrack. If you run low on air, there's a small supply at a cone at the entrance of the tunnel (2).
Head to the tunnel to the left of the ship, and open the gate glyph (3). On the first time through the tunnel, you will need to turn right, and get the key glyph again before circling around once again.
The route to the bottom has a strong current pushing Ecco away. To get past the current, sing to the sea turtle (4) to have it enter its shell, then push it onto the current. Follow the shell closely to avoid being pushed away, then swim to the right once you are far enough down. Head down and to the right to open the second key glyph (5), and swim all the way to the right.
This level ends when in a cutscene, where Ecco meets Trellia, a long-scale descendant of Ecco. She explains that she is from one possible future, and the other future involving the votrex. She transports you ahead in time.