Strategy Guide: Ecco the Dolphin - Perpetual Fluidity
Objective: Trigger the six switches to raise the water level and gain access to the exit door.
This level is a huge pain, but if you take your time and learn to recognize the landmarks, you should have no problem completing it.
After grabbing the Sonar power-up, get rid of the strange looking things partially blocking the tunnel. Watch out for the rotating blades in the tunnel (hug the walls as much as possible).
Once you reach the second room, dodge the security drones (the red lights move in a pattern) or blast them if your Sonar power-up hasn't expired yet. Head for the door at the far end of the room. In the next room, dodge the great white and head for the switch on the far side directly ahead of where you entered. Trigger the switch (it will turn green) and dash out to the right side of the room through the door you've just opened. Pass the security drones in the tunnel until you reach the room with the sharks. Don't bother fighting them now (they will help you recognize this room later), and exit through the door on the left side from where you came in.
After the room with the sharks, you'll be in a chamber with three security drones and a switch. Activate the switch and the water level will rise. Exit through the door, which is now accessible at the top of the room. There are a couple of sharks in the next room along with a switch. Ignore this switch and quickly exit through the red door directly below the one you came in. This next room has a bunch of crabs grouped together, and they may be hiding the switch. If they are, dash attack them, hit the switch, and exit the way you came in.
Now you should be back in the room with the two sharks and the un-triggered switch. Trigger the switch and exit the room through the green door near the surface. In the next room you should be aware of the piranhas. Hit the switch and head up and through the blue door halfway up to the surface. Dodge the two sharks and piranhas in this new room, hit the switch and quickly head back the way you came.
Back in the room with only the piranhas, hit the same switch again to turn it red, and go through the green door you used earlier to get to this area. Head straight through the next room into the white door and in the room after that swim down to the white door (not the one directly in front of you). In the next room, go through the yellow door to a room with two security drones and swim up to the white door near the surface, fight the current and you'll enter the last chamber. Use the Sonar power-up to break the red stuff blocking the exit and swim through it. If the water level isn't high enough, trigger the switch in the room.