Reverse osmosis is the method of forcing solvent from liquids using a suitable membrane. Just about all over the world drinking water purification systems include a reverse osmosis step. On board ship the sea water is pumped through the reverse osmosis system and pumped out as usable fresh water to be stored in the FW tanks. So the old problem of obtaining and using FW at sea is solved, with the system operating continuously the FW is used and replaced.
The system consists of :
- a series of sediment filters
- an activated carbon filter
- a reverse osmosis filter, which is the membrane
- a second carbon filter, to back up the membrane
- an ultra-violet lamp for disinfecting any microbes
So there it is potable fresh water from the sea, not exactly free of course but always available. For a full explanation of 'Reverse Osmosis' and the words of "The Rime of the Ancyent Marinere" visit Wikipedia.