With a fixed price, whoever comes in first gets the most water; laggards die. With a floating price, whoever has the most money gets the most water; poor people die. Neither of these is morally superior. IMO.
What if, instead, we were talking about bread? Assuming that rich people are fairly rare, then raising the price of bread high enough to be expensive but not so high that people cannot pull enough cash out of their pocket to pay for it, then the effect seems to me like it should be an approximation of a ration.
Clearly if Donald Trump gets to the store first and buys all the bread then nobody else would have any, but he could do that no matter the price. So long as the price is painful but feasible for anyone to afford, everything should work out more or less okay.
If the price is so high that nobody but Donald Trump can afford the bare minimum necessary, then there is certainly an issue, but I suspect raising prices that high would not be very effective.
(There might be some justification for putting in some extra-windfall tax, though. I am not sure.)
Tap water is nearly free and bottled water is >$1/gal. so the markup during a disaster is >100x.
Bread, fuel, generators, etc... these are all things we can afford to be relatively dismissive of and just say that price caps are unneeded/bad. That may also be the case with water, but I don't feel comfortable saying that with my limited knowledge.
Also consider that @ $1 per 16oz bottle you are paying $8/gal for essentially tapwater that was bottled and driven in from out of state. But try to sell a gallon of gas for $5 and you are some kind of monster...