If you have a dark surface with a film of water over it and you let sunlight fall on it you have a greenhouse. This is because the sunlight is absorbed by the dark surface and it heats up releasing significant infrared radiation which is absorbed within a mm or so of the film of water. Sunlight can pass through the film of water to the dark surface, but the infrared radiation released by the dark surface cannot exit the water film (it is absorbed too quickly).
So one could cool off cities by reflecting solar radiation, that would have heated the ground, onto a dark wall in the sea that has water pumped over it (creating a film of water over the dark wall). This will cause evaporation and enhance rainfall.
I must add this: One could have a huge matt black foam rubber sheet with holes in to let seawater onto the top and place it into the sea when needed. It could be attached to other sheets to make a huge artificial wet black floating island. It could be rolled up and stored on a ship.
