The floating solar islands concept could in theory supply the entire planet’s electrical utility needs. Assuming that the average incident solar power flux density on Earth is about 1 kilowatt/(meter exp 2), the Earth receives roughly 10 EXP 17 Watts of incident solar power. If only 0.01 percent of this power could be captured, the entire energy needs of the planet could in theory be met especially with improvement in efficiency of HVAC systems, transportation, home appliances etc..

Covering just 0.1 percent of the planets surface with 20% efficient solar cells would supply on average 10 EXP 13 watts given that half of the cells would on average be in the dark.

Solar energy thus produced could be used to power electric cars, trucks, and buses, home and building lighting, appliances, and HVAC systems. Biomass fermentation using ambient sun light and photo-bio-chemical processes could potentially produce ample supplies of methane and other hydrogenous gaseous fuels. Efficient fermentation processes under development could produce ample amounts of alcohols for use in hybrid alcohol combustion powered vehicles. For jet aircraft, methane powered rocket or jet systems could be developed which would not require petroleum based fuels to operate. Note that the Crew Exploration Vehicle or CEV being developed for NASA's planned return of humans to the Moon by the year 2020 will be able to burn Methane and Liquid Oxygen (LOX) for propulsion.

The prospect of going completely green may seem scary to some, but think of the clean air, carbon neutral energy supplies, and sustainability which could continue for as long as the Sun maintains its current power output; at least 500 million more years at almost the exact same output as the Sun's current output.

For those of you who feel that we are too entrenched into the petroleum industry and that complete change over to solar power and bio-fuels over the next half century will result in economic chaos and collapse, I ask you to envision the trillions, in fact perhaps 100s of trillions of dollars in revenue that could be generated in the R&D, manufacture, sale, distribution, and maintenance of such renewable energy systems.

As far as the future of space travel, the Sun puts out approximately 4 x (10 EXP 26) watts. Novel ways have been proposed and studied in academia which would use only a miniscule portion of this huge out put to enable manned interstellar space craft to reach very high relativistic gamma factors thus potentially opening up the entire Milky Way Galaxy for human space exploration and colonization.

Covering vast expanses of ocean with photovoltaic panels (islands) may well have the unintended consequence of upsetting the water vapor transportation cycle. Even a small amount can affect rainfall and ice formation elsewhere.
Power storage will remain the biggest hurdle for photovoltaic, whether the power utility or end consumer has to change out battery banks, the cost will be enormous and recurring. This is the single most costly factor preventing wide spread acceptance of solar power conversion. The world has been promised low cost lithographed photovoltaics dor decades, no consumer will see a cost/benefit when considering a retrofit that may be hundreds of thousands of dollars (all costs, conversions, consumables, maintainence).
I would be first in line for conversion if these issues were addressed so that the end user could actually see the promised savings in both cost and cleanliness.