On all boats larger than runabouts and daysailers, fresh water on board is both a convenience and a necessity.
Freshwater tanks can be be made from many materials but must be non-toxic, non-contaminating and taste-free. Rigid polyethylene tanks are available in hundreds of shapes and sizes, or you might use a flexible bladder tank--essentially a water bag.
Supply piping for a boat water system must also be non-toxic, non-contaminating and taste-free, and FDA approved for drinking water. If the system is pressurized or will carry hot water, the piping needs to be suitable.
Add a chemical purifier to your tank periodically to inhibit the growth of algae. The clarity of your water will improve and your system won't grow a permanent green lining. New UV sterilizers can also be added to kill micro-organisms and pathogens not affected by chemical treatments.
Boat owners who venture far offshore should consider adding a watermaker to their freshwater system. Use high quality hose and hose clamps and inspect the entire freshwater system frequently.