Somehow, you've got to find the source of octo-morphififying toxic waste! So far, your only lead is that industrial drainage pipe you noticed before you grew eight new legs. If you're going to find out what happened, why, and how you can get your real legs back, the only way to go is up.
You swim to the base of the waterfall created by the drain pipe. Between rocky outcrops and exposed roots of the above vegetation, there should be enough handholds for you to wrap on to. The slope isn't completely vertical, either, which should help, and hopefully you'll have enough strength to make it past the few stretches where you'll need to leave the water.
Well, here goes nothing! You climb out of the water underneath the base of the waterfall, using two tentacles as arms and letting the other six be your legs. Being out of the water makes you feel really heavy, but with effort you are able to reach up with your 'arms,' wrap on to an exposed tree root above, and climb up using a combination of pulling with your 'arms,' kicking up with some of your 'legs,' and suction-crawling up the slope with your other 'legs.' The industrial sewage waterfall makes a small puddle here, and you happily slip in to take a brief rest.