Nature based carbon offsets could migrate offshore in the 2020s, sequestering 3GTpa of CO2 for prices of $20-140/ton. In a more extreme case, if CO2 prices reached $400/ton, oceans could potentially decarbonize the whole world. This note outlines the opportunity in seaweed and kelp cultivation. It naturally integrates with maritime industries, such as offshore wind, offshore oil and shipping. Over 95% of the 30MTpa seaweed market today is in Asia, but Western companies are emerging.