My Question today is ,we live on an Island ,why can't the sea water be recycled instead of sewerage?

They are seriously considering this option.
In opposing however; in the long run, recycling water is more cost effective than a desalination plant. Also, by having a desalination plant, the waste water is returned to the environment in the same state as now, with a recycling system, all is broken down before it is returned to the environment. It is better money wise and environmentally.

Check out:
http://www.greenleft.org.au/2005/636/34165
http://www.deh.gov.au/minister/env/2005/mr07oct05.html
http://abc.net.au/news/australia/nsw/newcastle/200701/s1831148.htm

They both use very similar technologies, which is reverse osmosis or filtering the salts, microbes, chemicals and other undesirables out of the water. Seawater contains a massive amount more salt than wastewater, and salts exist in solution as ions, which makes it that much more difficult and expensive to filter them out of concentrated solutions. There are also higher corrosion and maintenance expenses associated with seawater, the filters need to be changed and cleaned more often (with more filters needed for the salt), as well as more power needed. So desalination costs up to 10 times more than recycling, with the same output at the end - clean water.

The only problem people have with recycling is that it might carry disease, whether microbial (easily removed from water), viral (much easier than salt to remove, since they are relatively large nucleic acids) or chemical, heavy metals hormones and such. The hormone or drug (and also viral) contamination is relatively easily removed as they are also large molecules and (in case of "filter failure" which people are also afraid of), also break down when added to open water sources, such as a dam. As previously noted, desalination also pumps the salty slurry back into the ocean (heavy metals and all), what good would that be for the environment around the outlet pipe?

Wastewater purification beats desalination on economic and environmental grounds. Many people just say "yuck, I don't know about it, but I don't wanna know".

Let face it, people have been drinking recycled water in one form or another for a long time. You can have it cleaned by nature or cleaned by a machine either way it is not NEW WATER. My waste toilet water goes down the pipe into the treatment system and out into the river and the next town/city who takes the water from the river or dam treads it again and repeats the process.
Desalination can be an answer to water shortages as is proposed in Sydney in the very near future. The downside is the cost, and the amount of electrical power required. So, how can we then generate a lot more electricity without generating more green house gas and other environmental problems?

I did neglect to mention that wastewater recycling does not actually add a net increase in water to the catchment system, at the moment only rain can do that. Groundwater is not replenished fast enough to be a long term solution. So, desalination does have a place in the Armageddon solution to add more to the system, but only alongside wastewater treatment.

Seems like a vicious circle,but lifes necessity.I am ignorant when it comes to chemistry and was very interested in your explanations.thank you