From Florida's sewage-stinking sands to Australia's dying reefs, the ocean is choking on its own overgrowth. Record marine heat waves and a flood of agricultural runoff are turning seaweed and algae—once the humble engines of life on Earth—into a global menace, smothering beaches, gutting fisheries, and carving out a dead zone in the Gulf the size of New Jersey. The good news: from kelp restoration to smarter farming, we already know how to turn the tide.