Fortunately, people on a seastead can easily use far less water than the average american [Economizing_on_Water_Use]. Folks living aboard their sailboats sure aren't using 150 liters of freshwater per person per day! Hill says:
People seem to use vastly differing amounts of water, some struggling on a gallon per day and others thriving on two pints. Pete and I manage quite succesfully on between five and seven gallons (19-26 L) per week between us, which amounts to a maximum of 4 pints (1.9L) per person per day. To achieve this, we don't seem to try very hard, but we do have water-saving methods, which make it easy and painless to be economical with our water use.This figure of 1.9 L/p/day is quite low, and we doubt seasteaders will be that frugal (unless they must). Rose suggests 7-20 L/p/day (2500-7300 L/p/yr) [Rose1979, p. 120] as a more typical level of water consumption. Unsurprisingly, these are all much less than on land. Requirements for drinking are 2L/p/day, more in hot environments or with strenuous exercise. Another source suggests 2.3 to 4.2 L/p/day as the minimum for drinking and 9 L/p/day as the minimum for hygeine, totalling 11 - 13 L/p/day [Eckart1996]. Jewell's closed-loop space station allocates 30 L/p/day for all uses [Jewell2001]. The ITDG suggests that in developing countries, the minimum requirement for personal use is 20 L/p/day, but that some functions can be performed with saltwater and a typical requirement for distilled water is 5 L/p/day. Source: http://seastead.org/commented/paper/infra.html#Fortunately_people_on_a_seastead_can_easily_use_fa
[Hill1993, p. 53]