No water at all other than melted ice, and only one layer of that?
There is a way. Melt some ice into water. Divide it into 2/7 (I think) divisions with some clever mechanics or micromanaged fill pond designations. Freeze that. (Possibly you have a subterranean reservoir filled with 2/7 water and pump that outside to freeze.) Melt it into 7/7 water. You can't mine it and still get water back, though. Rinse, repeat.
Difficult, I guess, but hey. You're not in this game because it's easy elvish, right?
EDIT: Thoughts as to how you could generate water.
Side view
##########m####
####### Xx ##P_
####_R__##_Xx #
###### xX ###^#
######_Xx #####
#########M#####
#= wall
Xx= pump, the X end being the output
^= pressure plate
This is based off the freezetrap elsewhere in the forums, of course.
So all the pumps are initially off. M has some magma in it. You designate P as a pond zone, to be filled with water.
The pressure plate senses 2/7 water and turns the pump on, dumping the water in the exposed area, where it freezes. Next, perhaps using mechanical timing, you switch on the magma circulator, which'll melt the ice into water. You will need a constructed, not natural magma tunnel where I marked m to melt the ice floor which will form above the water.
Then you run the last pump and suck up that water into the resevoir R. Mechanize all this and you won't even have to pay attention to it, it'll just keep on pumping out unlimited water.
Of course, all this is in theory. But it's a suggestion.
FURTHER THOUGHT: I think I figured out how to take dwarfs out of the equation.
Top down view
######
Xx.###
##,Xx,
Xx.###
######
#=wall
,=floor
.=channel
Xx=pump, X is the output
So the first pump pumps one tile of 7/7 water from the right hand side, which then splits and flows into the two channels to feed two water generators.