Saltwater algae oil is much more promising than ethanol from corn and sugar cane that require consumption of limited farmland, fertilizers, pesticides, and freshwater resources but I haven’t heard of anyone trying to use the open ocean to grow algae yet, they always try high capital cost/high efficiency methods. With the free open ocean high efficiency isn’t an issue since even 1% efficiency would capture huge amounts of energy. It would be hard to harvest but there’s probably a high oil fish that could do that for us.
I think I’ve read about that being tried but it didn’t work. They considered genetically engineering algae to be grown at sea, but there are some who are legitimately worried about super hardy mutant algae getting loose and taking over and causing us all sorts of problems. There has been a fair amount of experimentation with growing algae as a fuel crop in open ponds, and that didn’t turn out well. It’s too hard to control variables like temperature and competing wild algae and other contaminants in open ponds. Now most experiments are with closed systems growing salt water algae, algae that grows in brackish water, and fresh water varieties. None have produced anywhere close to the theoretical yields and none have come even remotely close to producing algae at a cost that would make it a commercially viable feedstock. This technology is not ready yet. It may work out someday and we may all be driving algae-diesel vehicles, or the idea may just fizzle out.