Big rivers like the amazon, Congo, Ganges, Ob and others discharge giant amounts of rainwater into the ocean. This water can irrigate all deserts but its just dumped into the ocean.
Globally average rainfall on land is higher than average evaporation. The amazon river alone could irrigate 15 million square km of deserts.
Why not sewing giant plastic water bags, the size of icebergs, and filling them with water right before its discharged into the ocean? It should have a minimal effect on the river ecology since the water already crossed it. The water bags could be slowly pulled by a small tugboat with minimal fuel use, compared to the cargo size.
The water discharged by the amazon would be worth $100.000 each second at current market prices in Dubai.