I suggest you wait with both these secrets, until you have finished Pumping Station Two.
When you arrive from Pumping Station Two, you have to push a button to create a bridge over some acid. Go back to the room you came from, and jump in the acid. Pick up the goodies, then swim to the start of the acid river.

Carry on straight down the acid river, as fast as possible, down to the end and turn right, down the slope and right again. FAST! Pick up goodies. On the way back, it's against the current. As soon as you can, get up on the left edge, jump into the acid lake below, and you are back at the level's starting point. Not a bad position for some backshooting.