For the map itself, Numbeo is not the primary source, that would be the WHO data, which is weighted 3 times more than the data coming from Numbeo to put an emphasis on a more reliable data source.

The cost-saving from not drinking bottled water is not necessarily dependent if the water is drinkable or not, as water filters are widely available and can offer substantial savings -> https://home.howstuffworks.com/save-money-with-water-filter1... The prices are taken from Numbeo and are the average prices for 1.5L bottles. I am aware this is not a perfect calculation method, I am planning to add a cost-saving calculator soon which will offer more flexibility.

Regarding the reputable source of information, you are right. Country-level data can never be a good decision-tool to chose if you can drink tapwater somewhere or not. It has to happen on an area-basis and include scientific reports, which are hard to get as of 2020 as there is no single source/database/API for that, except for very few countries (including the US and Austria), I'm working on that and I'll try to improve the UI so it reflects that one should check the water quality in the area of interest rather than the country itself.