Your membranes are plumbed in series. The flow direction and order you've described is correct. But you need to verify the high pressure side first of the intensifier, this is the side that feeds the first membrane. Also notice when you remove the membranes the placement of the membranes brine seal and put the new ones back in the same order. The brine seal side of the membrane must be on the feed side or input side of each membrane vessel. This is important because of the design of the brine seals
lip. There should already be a 5-Micron pre-filter installed. You can add a 20 Micron in series if you like. On the higher production models it's not a bad idea because of the increased flow. Every harbor is different, I would not recommend operating a watermaker
in a dirty harbor no matter how good a pre-filter system is on board. Use the watermaker when you are out cruising and grab a hose when you are in harbor. Also before you invest in new membranes you need to check that the intensifier and feed pump/s are producing the correct pressures and flows first. Shenker will have the specs and procedure for your particular model.