Actually, we don't. Since you can't kill the blasted thing.
You have the speargun and the charges, right? That being the case, first blast those kraken's minions out of the water, and when the coast look clear enough, swim a bit closer (but not TOO close), enter the first person aiming mode and shoot the octi in one of its eyes (those yellow dots around its mouth) this will make the thing to retreat. Now, there should be four pillars near the place the kraken was before pulling back. Swim near one and set a charge. The thing returns, but one of the pillars is gone (or two, if you've been fast).
Repeat the thing above 'til you have blasted all the four pillars... and you can enjoy the ending cinematic of the level.
