The algae do not produce hydrogen naturally, but only as a result of being stressed. The system must be restored to normal non-hydrogen photosynthesis every few days to recover or the algae die. The algae must be exposed to sunlight to grow, yet be enclosed if the hydrogen is going to be recovered. This makes "open ocean" use doubtful, and would probably required many acres of some kind of greenhouses, with the equipment needed to maintain hundreds of tons of aquatic plants.

I would bet a lot of money that taking the same acreage devoted to these algae greenhouses, and devoting it to solar panels hooked up to an electrolysis system, would be much more productive and efficient. In addition, the cells, unlike the algae, can be hooked directly into an electrical grid.