"Reverse osmosis (RO) seawater desalination is currently a...

"Reverse osmosis (RO) seawater desalination is currently a widespread means of closing the gap between supply and demand for potable water in arid regions. Currently, one of the main setbacks of RO operation is fouling, which hinders membrane performance and induces pressure loss, thereby reducing system efficiency.

An alternative water source is saline groundwater with salinity close to seawater, pumped from beach wells in coastal aquifers which penetrate beneath the freshwater-seawater interface. In this research, we studied the potential use of saline groundwater of the coastal aquifer as feed water for desalination in comparison to seawater using fieldwork and laboratory approaches. The chemistry, microbiology and physical properties of saline groundwater were characterized and compared with seawater.

Additionally, reverse osmosis desalination experiments in a cross flow system were performed, evaluating the permeate flux, salt rejection and fouling propensities of the different water types. Our results indicated that saline groundwater was significantly favored over seawater as a feed source in terms of chemical composition, microorganism content, silt density and fouling potential, and exhibited better desalination performance with less flux decline.

Saline groundwater may be a better water source for desalination by RO due to lower fouling potential, and reduced pretreatment costs."

http://pubs.acs.org/doi/abs/10.1021/acs.est.5b03634

This probably explains why CWE has been drawing saline water from the coastal aquifer at Garden Island.

I would think it would be easier to maintain and service this type system than one that is located several kilometres off shore and on the ocean floor.

That would have to be a consideration for remote island locations that have limited facilities.