Tap water in the US costs on average about $0.01 per gallon or less. People typically drink a gallon or less per day, so about $0.30 per month. Your water bill is pretty much not affected by the tap water you drink, just the water you use for everything else.
Bottled water is easily hundreds of times the cost.
Why are you comparing $1 to $0.25? This is an incorrect way to compare relative purchasing power.
As already pointed out, if $1 in 1938 is equivalent to $21.77 today, then $0.25 in 1938 is equivalent to $5.44 today ($21.76 / 4). Since minimum wage is $7.25, they are earning more per hour now after adjusting for inflation.
Another way to think about it is if someone wanted to buy something for $1 in 1938, they'd need 4 hours of minimum wage work ($1 / $0.25 = 4 hrs). That same $1 expense would be $21.77 today, or $21.77/7.25 = 3.0 hours of minimum wage work.
This isn't necessarily justification that the minimum wage isn't in need of an increase today, by the way. I personally think it needs an increase (among other work reforms) and is a decent argument that minimum wage in the US has been too low since it's inception. But it has increased since 1938 after adjusting for inflation.