Desalination of sea water is usually done
with one of the following methods: reverse osmosis or distillation. Reverse osmosis is where salt water is forced against a filter. The filter has extremely small openings, so small that only water can go through it and the salt is left behind. Try looking up osmosis in a biology textbook. Desalination uses the reverse process, so it takes a lot of energy to force this to happen.
When water is distilled, it is boiled and the steam is collected. Since steam is just water that's been turned into a gas, if you cool it down, it reforms water. Salt doesn't get carried away with the water, so you get pure water. You may have seen this if you've made pasta with salt water and left it too long. All of the salt is left in the pot and the water is evaporated. If you could collect the steam from the pot, it wouldn't be salty.
Both forms of desalination take a lot of
energy (usually in the form of electricity) so it is very expensive to produce drinking water with them. This is the largest con.
Santa Barbara actually has a reverse osmosis desalination plant, but it isn't used because it makes the water too expensive to buy - it's for emergency use only. The benefit is that if you have a vast supply of cheap renewable energy, there is a huge body of water- the ocean - that we can draw water for drinking from.