Breaking apart H2O with solar energy sounds clean, but it requires more energy input than you get from burning the hydrogen after you isolate it.
Not only that, since hydrogen is the least dense element, transporting and storing it also has huge costs.
It either has to be compressed at extreme pressures or liquefied.
Since hydrogen is explosive, that is not great idea, either.
If they can bioengineer kelp/seaweed to produce air sacs of hydrogen from sunlight and salt water, we’d be set.