Swim forwards, hugging the wall to your left, and take your first left when possible. Swim forwards again and then take a right when you reach the wall in front of you – be careful to avoid the giant rotating mechanism, which will kill you dead. Continue swimming straight, hugging the wall to the right until you’re again able to turn right. Keep swimming straight on until you reach a PORTABLE AIR on the ground.
Hug the floor and spin 180 degrees, swim forwards a few strokes until you’re able to swim down into the opening. Head straight down, and then swim forwards (as in, the opposite direction to where you just picked up the last item). You will see an item on the floor – swim down to collect the MAGNUM SHELL. Again, be careful to avoid the rotating mechanism.
Swim around to the left and then straight forwards until you reach another wall in front of you and turn left – again avoiding another rotating mechanism – and pickup another PORTABLE AIR.
Continue forwards avoiding the trap on the floor – which will suck you down and chop you to bits – until you reach another wall and pickup the 1750 CREDITS, avoiding yet another rotating mechanism.
Turn left and swim forwards all the way to the wall in front of you before taking another left. Swim forwards all the way to the wall in front of you, turn right and then swim forwards until Acarno is out of the underground waterway.