Saltwater could be used for those purposes, but it would require a dual water distribution system which would cost a fortune to build and maintain, and it would create a cross-connection nightmare. Not to mention the effect it may have on the ability of the sewage treatment system to work - that's a biological process and I'm not sure what the effect of high salinity would be.
I don't think salt water can be used for irrigation - will kill most plants and trash the soil, and mess up the groundwater system - on an island like this (and many others), fresh groundwater floats on top of the denser seawater in what's called a "lens" (Ghyben-Herzberg Lens). It's a pretty precarious balance, and even over-pumping a well can disrupt the lens in a local area for years. Dumping seawater in from above, thorugh irrigation or even de-salinization waste, does the same thing or worse.
De-salination on the other hand is god-awful expensive at the public water supply level. We all drink de-sal'ed bottled water here already, produced from moderate salinity groundwater. Because water from the lens system is not really fresh - it still has ~500 ppm chlorides, at best, which makes it taste like crap. Desalinizing moderately saline groundwater is economical at a bottled-water scale. Dealinizing seawater is extremely energy intensive - most of the big hotel resorts here do that.
