We have two 190 L water tanks, each of which lasts us a bit longer than a week (with the two of us). That means we use roughly 10 L per person a day. That includes a short shower, washing dishes, drink water, cleaning and cooking. We also have a 20 L jerrycan which we try to fill up when we go into town.
If we were to cross an ocean with a crew of 4, we would definitely have to cut down on showers. If we half our water usage, we still don’t have enough capacity for three weeks. So we plan on buying a 100 L* water bladder which is connected to the drink water system. We could also take additional jerry cans or bottled water for emergencies (leak in a tank, contaminated water, etc). If we get the water supply on board close to 480 litres, we will have 20 days with 6 L/person. Sounds just about doable!
Many cruisers who are in it for the long haul consider buying a water maker. That would be of course great, but it uses a lot of power.. Which brings us to the next topic: III. Electricity.
*Edit: We bought a 150 L water bladder, which brings our total up to 530 L. We are also planning to take bottled water, milk and cans (soda, beer). That should bring us across!