Every cyclist has water bottles mounted to their bike frame. That’s fine for short journeys between towns but you’ll need to carry far more than a couple liters if you want to wild camp or plan on crossing remote territories like deserts.
The cheapest option is to buy water in plastic bottles from a supermarket and then reuse the bottles as needed until they wear out or your trip ends. This has a few advantages. There’s a very low cost to you and the bottles can be found anywhere. On the other hand, plastic water bottles are very wasteful, can break if they fall off the bike and are sometimes awkward to strap on your panniers securely.
An alternative is to buy a reusable carrier designed to hold water.
We started out with just such a bag from Platypus but it broke down within a few months and had to be replaced. The second time around we went for a product from Camelbak, the Unbottle 70. Like the Platypus bags, the Camelbak water holder is made from reinforced plastic but also comes with a protective cloth pouch around the water bag. We bought two, holding two liters each.
There’s a large mouth to pour or filter water into and a drinking tube with a secure valve to stop water leaking out when you’re not using it. We also like the clips on the side of the bag, which help us secure to the bike.
Another option to investigate is a water carrier from Ortlieb. They come big enough to hold 10 litres, easily enough for two people to cook and clean with for one night of wild camping. This amount of water is great if you plan to take an al fresco shower in your campsite but the downside is figuring out a way to mount the bag on your bike. Because the water is in a single bag, its weight can’t be shared between riders or distributed evenly across one bike.
