This article mentions that giant oil tankers use seawater ballast for their return trips. Substituting fresh water, or even processed sewage would seem to be beneficial if taken to desert areas.

http://www.think.cz/issue/40/hereonearth40.html

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"The oil supertanker unloading at the Venice oil terminal can backload fresh water pumped from the river Po. 80 million gallons of freshwater ballast, can fertilise and irrigate desert lands. We can use water as a by-product for what we are trying to achieve, the reduction of global warming. As the water evaporates it will cool the overheated air, refertilise the deserts, cut rising water levels in European cities, and lower pollution levels in the seas. On the most marginal lands, in the most impossible environments, against the most appalling odds, it is here proposed the building of a desert city, to fertilise and irrigate deserts using the waste water of European rivers.

SOMEWHERE TO START:
The emptiest countries in the world surround the greatest deserts. The population of Mauritania has migrated to the coast in front of the ever-expanding Sahara. Their farms abandoned to the sand, they have turned to fishing the Atlantic. The northern port, constructed for the export of iron ore, anchors a railway line running nearly 1000 miles into the Sahara. "

I have had my eye on the Amazon River for some time as a potential fresh water source for turning the Sahara into a big truck garden. With nuclear energy sources, maybe the costs of water transport would be low enough?

People from Bangladesh, and south Seas Islanders flooded out could be invited to settle there, in the Sahara, to collect camel dung for fertilizer, and to raise hemp.