Craig & Katherine Briggs SN 68 Sangaris Tropic Isle Harbor, FL
Regarding brackish water operation, that's one of those topics that's been burdened with a lot of myth and misunderstanding in the cruising community. It is absolutely OK to run your water maker in brackish or even totally fresh water. It will not damage the membrane as long as you do not exceed the membrane's rated product flow.
Think of it this way - the "green" zone is the pressure at which the membrane is passing it's rated flow of product water. If that is exceeded the membrane will start to tear apart and fail. Our pressure gauges happen to show a "green" zone for sea water and that's in the range of 800-850 psi / 55-58 bars which gives you your rated output for sea water. However, the "green" zone for water of lower salinity is at a lower pressure. So, for example, the fresh water "green zone" is only about 150-200 psi / 10-14 bars for the same rated output, and the brackish water "green zone" will be somewhere in between, depending on its salinity.
The proper procedure for brackish or fresh water, then, is to bring the pressure up slowly until you are getting the rated flow of product water. Don't crank it up to the sea-water "green" zone or, yes, you will damage the membrane. And running sea water at 950 psi (65 bar) may shorten the life of your membranes, although that may be OK in the Med with it's very high salinity vs., say, the open Pacific or Atlantic as long as the product flow is correct.
I'd guess the idea that running your water maker in brackish water will damage the membrane probably stems from having our pressure gauges show a sea-water "green zone" - some people have cranked the pressure up to that "green zone" in brackish water and damaged their membranes and hence the misinformation starts to circulate.
Bottom line though - feel free to sail up the Guadiana or lock into Loch Ness or sail the Great Lakes - you can make water in any salinity - just don't crank the pressure up to the sea water green zone.
Craig, SN68 Sangaris