Desalination isn't particularly hard at a small scale.

The issues with desalination - the area around the plant gets saltier than the rest of the ocean (you have to dump that excess salt somewhere,) which kills all the marine life in the vicinity of the plant. Also, humans need a ton of water, so the plants need to be very large to fulfill all the domestic demand. And since the plants are at sea level, you need large pumps to get all that desalinated water into pressurized pipes - which means you need a lot of power.