1) It is not as well regulated as your municipal drinking water.
Municipal drinking water is tested on a regular basis thanks to the Clean Water Act we have in Canada. Compared to bottled water companies, we test our water 4 times or more than the average water bottle company, meaning the quality of the water is much more likely.
2) Bottled water is expensive.
Even the cheapest bottle of water will cost you an arm and a leg over time. For 1000L of bottled water you are looking at about $1500. For 1000L of tap water, $1.26.
3) Bottled water is wasteful.
If you drank 3 standard personal
sized water bottles per day for a year, you would have enough empty water
bottles to lay end to end across 8 and a half basketball courts....just you.
4) Plastic water bottles can be bad for your health
This is especially true if you reuse them. When a water bottle is exposed to extreme temperate like cold (freezing) or hot (microwaving or washing is hot water), it can start to release some of the chemicals from the lining like Bisphenal A (BPA) which can be harmful to your health.
There are many other reasons I believe tap water and a good filter are better for your health, but it would make this post far too long. Here is a great clip from Pen and Teller's TV show Bullshit which sums a lot of it up nicely. I should warn you as always Pen has a habit of swearing.