A water maker may be factory or aftermarket installed. The figure you refer to may be common for a factory install.

If you visit the water maker manufacturer web sites you will see much higher options. For example, our watermaker makes over 40 gallons per hour. That's around 3 x the volume.

Assume that each person uses as much as 10 gallons of water per day. If there are 8 people on board. You may need 80 gallons of water per day. That may take up to 6 hours of running time on the water maker.

Some may argue that 5 gallons per day is more realistic. However, when a water maker is on board, nobody is a water miser and consumption increases dramatically. Go to tell the crew not to run the taps and to keep showers short. People may think you are stupid because "there is a water maker" on board and disregard you.

Even if you consume 5 gallons of water per person per day, the watermaker would still need to run 3 hours a day to keep up.