All oil originally comes from saltwater algae so this is not far fetched, is actually step one of the process. All coal comes from freshwater plants.
The open ocean is 70% of Earth's solar energy absorbing surface, and most of it is underemployed. The main problem with using the open ocean vs. using saltwater ponds is algae species contamination, and the higher cost of piping extra CO2 in to make the algae grow.
I'm sure Lavoisier and equivalent exchange will dictate something has to be sacrificed in order of this to work.
Nothing is free, we often create more of a problem than a solution.