I've just pledged and joined Travelers Against Plastic, a non-profit encouraging travelers to reduce the use of plastic water bottles. Living along the False Bay coastline, I can't tell you how discouraging it is to see plastic waste wash ashore. In addition, the plastic bottle creation process is taxing our environment far more than it should. You can get up to speed with all the reasons why buying and tossing plastic bottles is harmful to our environment and join me with a pledge to reduce your use of plastic water bottles on the Travelers Against Plastic website.
So, how can you stay hydrated while avoiding purchasing plastics? The first option is water purification. Tablets are the cheapest option to rid local tap water of nasty bacteria. Personally, I don't like the taste of the water after using them. Other options are available, like SteriPEN (Google Affiliate Ad), which uses UV light to purify quickly (and it's USB chargeable? Yes, please!)
The second option is to carry a reusable water bottle with you. I'm a fan of Kleen Kanteen's wide mouth bottles (Google Affiliate Ad) because they're water filter compatible. Keep the bottle in the refrigerator overnight and a cool, refreshing drink awaits you the next morning.
If the tap water is not safe to drink where you're traveling and you can't purify it, when you arrive at your destination, buy one giant bottle of water (like 3 gallons) as supply. You can refill your smaller reusable bottle from the big bottle each morning. It's so easy, you'll never go back to buying plastic bottles while traveling again.