Watermakers allow for greater self-sufficiency onboard however they are a drain on electrical systems and a costly piece of equipment that require maintenance, filters and space to install. Most watermakers use reverse osmosis desalination, see the image below for an example. 

Some watermakers can operate at 12 watts per gallon on DC power or through an AC converter. Larger units can run on AC power or belt driven off your engine. It is possible to run them from solar and wind powered systems.  They can make 5 to 50 gallons (for non-commercial models) of water per hour which can make life on board quite nice. 

Don't forget the manual units for life rafts.

• Increases fresh water useage
• No worries about tank purity
• Bring on the showers and clothes washing!
• Self-sufficiency 
• Requires DC, AC or belt drive
• Requires maintenance
• Prices vary, but be prepared
• Requires space to install

Pictures of watermakers