I propose floating solar arrays that use the sun's rays to generate electricity and that is immediately used to power electrolysis and create 2 very powerful fuel sources. If we design them to be self righting and come up with a good way to harvest en-masse, then we could just create them by the millions, throw them out to sea, and then harvest them after they've ripened (i.e. filled up their tanks). They' have to be tough to survive the ocean and harvesting process, but if a potato can be pulled from the ground by steel teeth, I'm sure we can keep these devices intact long enough to harvest the gas.
Also, bubbling CO2 (or air with lots of CO2) in an aquarium will bring on lots of algae...so let it happen in controlled situations, harvest the algae, bury it or convert it to hydrocarbons again.
By back of the envelope calculations (which require some highly unscientific guesswork), I'd say that a solar + hydrogen roundtrip could have an optimistic long term efficiency goal of 26%. Once you factor in manufacturing, deployment and harvesting at sea, it becomes quickly unsustainable.