Haha, funny! I actually did exactly that in a real case.
The approach is basically like any other market sizing question. A key aspect of bottled water markets is that you will need to understand that there actually different markets:
- The most intuitive one is the market for small bottles, let's say below 2 liters. Typical end customers are households and companies, public sector facilities, etc. that offer bottled water to their employees, etc.
- Another market on top is the market for 5 gallon canisters that you will often see in a B2B setting, often with a paper cup holder next to it. Offices, clinics, etc.
Given these two markets, you can split the work into a B2C market (small bottles, in-home or on the road consumption) and a B2B market. You can approach the first one building on population size and estimating consumption per day.
For the B2B market you'll need a bit more sophisticated approach, e.g. estimating the total number of clinics, offices, etc that are customers and then calculate the size from there.
Hope this helps to build the right approach.