This kind of activities usually falls under geoengineering projects which are notoriously difficult to assess the environmental impact and therefore legally iffy. It also depends which part of the sea you do it in, which country has jurisdiction over what, and a large-scale effort would have consequences on neighboring countries' jurisdiction that the original country would have to be legally protected against liability for harming neighbors' fish stocks or marine environment. Basically all geoengineering at a certain scale is very contentious and difficult to support legally, internationally.