This reminds me of Maelstrom by Peter Watts:

Lubin stopped a couple of meters from the animal, holding a length of driftwood as thick as his forearm, carefully splintered to a point at one end. The stink was appalling. Maggots squirmed in every sore.

Lubin laid the point of his weapon on the back of the animal's neck.

"Hi," he said softly, and jammed it home.

Amazingly, it still had strength to fight. It reared up, roaring, caught Lubin in the chest with the side of its head, knocked him effortlessly into the air. Black skin, stretched across the twisted ruin of the lower jaw, split on impact. Pus sprayed from the breach. The bull's roar slid across the scale from defiance to agony.

Lubin hit the shore rolling, came up safely outside the sealion's attack radius. The animal had hooked its upper jaw around the shaft embedded in its neck, and was trying to dislodge it. Lubin circled, came up from behind. The bull saw him coming, wheeled clumsily like a battered tank. Lubin feinted; the bull charged weakly to the left. Lubin spun back, jumped, grabbed: the wood sent splinters into his palms as he jammed it down with all of his weight.

The bull rolled screaming onto its back, pinning one of Lubin's legs under a body that—even at half its normal weight—could still crush a man. A monstrous face, full of pain and infection, lunged at him like a battering ram.

He struck at the base of the jaw, felt bone tearing through flesh. Some deep pocket of corruption burst in his face like a stinking geyser.

The battering ram was gone. The weight shifted from his leg. Thalidomide limbs flailed at the gravel by Lubin's face.

The next time he got the spear he hung on to it, pulled from side to side, felt the deep scrape of wood over bone. The bull heaved and bucked beneath him; in a confusion of agony from so many sources, it didn't seem to know where its tormentor was. Suddenly the point slid into a groove between cervical vertebrae. Once more, with all the strength left to him, Lubin pushed.

Just like that, the heaving mass beneath him went limp.

It wasn't completely dead. Its eye still followed him, dull and resigned as he circled the animal's head. He'd merely paralyzed it from the neck down, deprived it of breath and motion. A diving mammal. Adapted over how many millions of years to survive extended periods without breathing? How long would it take that eye to stop moving?

He had an answer. Sealions were just like other mammals in any number of ways. They had that opening at the base of the skull, that place where the spinal cord climbed up into the brain. The foramen magnum, it was called; such anatomical tidbits were always coming in handy to people in Lubin's line of work.

He pulled his weapon free of the flesh and repositioned it near the back of the skull.

The eye stopped moving about three seconds later.