early items: gravity boost before boost ball
In Hydrodynamo Station, enter the water and move around the ring until you find the light or dark door. Line yourself up and jump off the ring, and once you fall under it, use your second jump to do an underwater dash. Timing is important here; too soon and you'll collide with the ring, too late and your second jump won't get used.
Once you land by the door, scan the console and take the kinetic cannon back to the top of the room. Repeat everything you did with the other door this time, and after scanning the second console, scan the third near the bottom of the room. Now you can go down to where the gravity boost is.