Let's talk about water bottles. Water, in general, seems to be quite a confusing subject. Should I buy bottled water? Is it safer than tap water? What about filtered water? This is something you'll have to determine based on the quality of your tap water. Keep in mind, however, that bottled water is less strictly regulated than tap. I, personally, have rusty well water that with a small filtration system (such as Brita) tastes and tests just fine.
Disposable water bottles are extremely bad for your health as well as for the planet. Let's take the planet first. Making, bottling, packaging and shipping of disposable water bottles means the usage of nonrenewable fossil fuels and adds to the pollution and greenhouse gases never mind the fact that most aren't recyclable and end up in the landfill. Definitely not green.
As far as the harm to your health goes, studies have shown for quite some time the ill effects plastics can have. We know that reusing plastics not meant for that purpose can mean exposure to the chemical BPA (Bisphenol A), known to interfere with hormonal development.
Bottles made for one-time use only are thin and most likely made of PET (Polyethylene Terephthalate). They have the possibility of building up germs and the plastic begins to break down with each re-use. Additionally, you never want to leave plastic bottles in the sun or high heat. The chemicals will leach into the water much quicker, which in turn, you ingest. Unfortunately, most often these are the disposable bottles that are actually recyclable so people think it's a good choice.
Then there are plastic bottles made from Polycarbonate, such as nalgene bottles. There is the possibility these too can leach BPA into what you store in them. More recently are bottles made of HDPE (High-Density Polyethelene), such as opaque nalgene bottles. They are currently listed as cautiously safe. If you choose to use any type of plastic bottles use them with care. Clean by hand, don't put them in the dishwasher, and when they become scratched replace them with a water bottle made from stainless steel or glass.
The safest bottles to date are those made of stainless steel or glass. There are no ill side effects to drinking liquids from these materials. If you are considering purchasing bottles that are stainless steel, make sure they aren't lined as the lining has the potential of being dangerous.
Making this small change will have a positive impact on both your health and the planet. The largest impact we can make collectively is for all of us to make small choices everyday. We can all strive to achieve light shades of green.