You have two options, fresh water rinse every week, or pickle it.
It’s most likely pickled and most like done with the Sodium meta whatever.
If I were you, I’d fresh water flush the heck out of it and then let it make quite a bit of water, read the directions, but basically if your in brackish water you keep the pressure low enough so that your not making more water than it’s rated to make.
After I was sure everything was working good, I’d pickle it with the RV antifreeze
, it’s much kinder on pump seals
and last much longer.
Only downside to it that I know of is that it’s harder to carry a supply with you as it takes up more space than the sodium meta whatever.
Who makes your watermaker? Maybe give them a call and ask their advice?
I’d run it to find out if everything is as it should be so I’d know what if anything needs fixing while you have plenty of time.
I’d plan on having to replace the membranes, I’m not saying to do it for sure, but expect that two years from now you may have to. Just because you don’t know their history