Robert, speaking at the World AirlinesForum on sustainable development in Cannes in 2007, Airbus director of sustainable development Rainer von Wrede said that there are no technology obstacles -to the development of hydrogen-fuelled aircraft engines - but there are many hurdles, including the ability to produce hydrogen in sufficient quantities and in an environmentally friendly way. "I would say it would take around three decades to solve these problems," he said.
It is OTEC's potential to develop hydrogen in quantify, and in an envrionmentally friendly way, that makes me an advocate.
As to your concerns about the greenhouse effect of vapor, the European Union points out, "the residency time of water vapour in the upper atmosphere is 6 months, whereas carbon dioxide remains in place for around 100 years."
In their paper, H2Aircraft - CRYOPLANE and the future of flight, they state, "Driven by the foreseeable exhaustion of petroleum reserves and tightening environmental controls, the EU-funded CRYOPLANE project has developed a conceptual basis for a new generation of aircraft, preparing the way for a future without fossil fuels and where hydrogen rules."