People have been growing coppice forests with salt tolerant trees such as willow for ages. No need to desalinate, the trees do it themselves. You can also use

People have been growing coppice forests with salt tolerant trees such as willow for ages. No need to desalinate, the trees do it themselves. You can also use algae, which can grow in salt water. The fusion energy that is powering the system is that of our sun. No perpetual motion machine required, our planet is being bathed in outside energy.