Are you looking to purify/filter your fresh water? I installed a carbon filter under my galley sink that works quite well.
If you'll be making fresh water from salt water, you'll need a reverse osmosis water maker. These are expensive, but nice to have for extended cruising in remote locations. Some are now made in compact, modular units. The trick with them is to use them consistently and do the maintenance, (back flushing, etc.), or the membrane can get clogged. Also, only run the watermaker in water that is free of sediment or pollutants. Good luck!