The objectives for Aquarius Falls are tricky. Make sure you blast every trigger you see. It may be vital.
And watch the robot generator. It produces twice the normal amount of robots.
You must obtain a Blue Key from an ECM dropping Smart Mines. Note that once you're through the Blue Door, there's no way out. Blow another ECM containing a Yellow Key, then blast the switches behind the Blue and Yellow doors. Go back to the robot generator and towards the waterfall at the back. Behind a grate there is an easily visible Yellow Door, behind which is the next trigger.
Go back to the upper Blue Door. There is a grate nearby which has been opened. Fly through this to a room with two pieces of rock jutting out from the ceiling and floor. At the end of these are two triggers. Destroy both.
In the previous room there was a door with a grate in front of it. This grate has now been removed. Find a Yellow Door with a Mini-Reactor behind it. Destroy this and another Mini Reactor in the room which another trigger is behind. Blast it and collect the Red Key, then blow the reactor. The Guide-bot will lead you to the exit.