The cow burp effect is a myth. With five minutes thinking about it, is easy to see why is a no solution. I have explained here before why will not work.
Most of the seaweeds are water. And the dry matter is salty.
Coastal areas are extremely busy. You can't remove Honk Kong city to culture seaweeds. You can't block maritime traffic routes to culture seaweeds, or stop all commercial fishing near the shore, or ban tourism. In most areas is a no-way.
And there are other problems and obstacles that could be spotted easily by biologists, for example that water temperature is rising.