Trellia's Bay is the location where Ecco appears after being transported to the good future, populated with more Skyfins. Ecco will need to reach the Skyways.
Ecco will need to exit to the left, and reach the surface. There are seaweeds in the route, which will slow down any fast movement. Beyond the surface are some floating islands, and water tubes that head to the sky, but the exit is blocked.
Ecco first needs to reach the glyph found in the center floating island (marked as #1 in the map). It's easiest to approach by ascending the right-most water tube, and either jumping across to the adjacent tube, or to get assistance from the Skyfin and dropping when above the island. Touching the glyph will remove a stone barrier.
Return to the ocean, and reach the previously blocked glpyh near the left of the area (#2). Touching it will open a glyph to the right, for a short period of time, thus quickly swim to the right, jump over the island and enter the cave. Touch the glyph at the back. Even if time runs out, the glyph allows Ecco to leave the cave.
After touching the key glyph, exit the level. It can be reached from the same right-hand waterway, jumping one tube to the left, and removing the blocking glyph.