For the water, the simplest solution is to have wells in every prison cell.
For the food supply, a clever combination of a looping track, dumping holes over food stockpiles in the prisons, and linked supply stockpiles should do the trick. If you make some number of small (1x2 or 1x3) no-barrels plump helmet stockpiles, where each stockpile takes supplies from all of the stockpiles before it (but none after it) then the stockpiles should get filled up in order from the last stop up to the first. Once the first stockpile gets enough it gets loaded into the minecart, which then dumps down the hole into the prison, then continues down through each stockpile, loading and dumping, until you get to the last one and loop back up to the start.
You could add a garbage dump zone next to a hole in the floor of the prison that empties out over a seed stockpile, if you want to recover the plump helmet spawn from the prisoners (you'll have to manually unforbid them after they're dumped though).
So a possible prison cell layout would look like this:
XXXXXXX
XXXTXXX
XX.C.XX
XWBRGPX
XX.F.XX
XXXXXXX
X = Wall
T = Table
. = Floor (or whatever else you want)
C = Chair
W = Well
B = Bed
R = Restraint
G = Garbage dump zone
P = Pit over seed stockpile 1 Z-level down
F = Food stockpile (plump helmets dumped here from above)
Of course I'm being nice and giving them beds for sleeping and chairs and tables for eating (another advantage is that you could make one of them into your bookkeeper by turning the cell into an office).