TheGenerator says:
... I hope I'm not making things too difficult ...
No, its fine. It is not difficult, just complicated.
TheGenerator says:
... not as much a miracle, but more like the D&D spell "Purify Food & Drink" ...
That is a DnD problem. Outside of DnD that exact act is one of the most common examples of a miracle. Healing and feeding are the big ones.
DnD trivialising those and makes all sorts of playstyles/gamestyles impossible. How can one run a 'survival' game when first level spells like Goodberry can completely feed, water, and sustain ten people for a whole day (in
addition to the healing benefits), and those Purify spells can make tons of anything that can be considered 'food' safe. At least with Purify, it first has to
be 'food or water' --so something you would already consider drinking but was maybe a bit risky-- and it does not say it makes it any less unpleasant.
TheGenerator says:
... divine spells
are miracles? ...
Pretty much. We probably have miracles and spirit manipulation, people
like miracles.
The fact that you need to roll for it each time takes this away from the DnD system of 'guaranteed 10 fed people per day' to 'this might work if my god wills it' type miracle territory. And will need an unlikely result (maybe 5 or 6 if you want to roll the Die of Fate, 10 or 12 if you want to roll +STAT?) to work.
Unanswered petitions for miracles possibly comes with irritated gods as the downside. Getting sick from drinking the water was already happening, and is a consequence of drinking the water, not of the failed miracle.
Someone should roll this so we can see how it turns out.