Staying hydrated is important. Especially when hiking and walking long distances. In general, a person should consume about 2 liters of water a day. During a 15-20 kilometer hike, that requirement increases. And, if it's a hot day or the terrain is steep or difficult, the number goes up even more.
It's also worth noting that by the time you're actually thirsty, you're already dehydrated. So, getting in the habit of drinking water while walking is incredibly important. (Perhaps I stress this a bit heavy because I live in the desert and water truly is life when out in the wild here!)
The availability of fresh, clean drinking water along the Camino ways is one of the big reasons they are safe to travel throughout the year. You will find water sources easily on most of your walking days. (There are several days along the Via de La Plata where you have to carry a bit more water because there isn't any available.)
While water is heavy (one liter weighs one kilogram), it's essential to carry enough to keep yourself hydrated, healthy, and safe. And, there are two ways most pilgrims carry water: bottles and bladders.
My personal choice were bottles. I love Camelbak water bottles. I had two, 1 liter bottles. They fit perfectly in the outside, side pockets of my Osprey backpack, so I had easy access all the time to them. The ring on the top also allowed me to use a carabiner clip and attach an "in use" bottle to the front straps of my packpack, making for even easier access.
Why did I prefer bottles? They're easy to refill throughout the day and they're easy to keep clean inside and out.
The second choice is a bladder. Most of my fellow walkers chose bladders. Why? They hold more water than a bottle and they attach inside the backpack, allowing them to stay cooler and allowing for their weight to be more evenly distributed via the backpacks basic structure. Of course, they aren't easy to refill during your hiking day and they can be difficult to clean.
As with all Camino gear, the choice is yours. This is one of those items that you should definitely practice with before hitting your Camino. Perhaps that's why I like the bottles so much. During my weekly hiking adventures, I fill the bottles and tuck them into the pockets of my backpack. I use them all the time. If I had started with bladders, they might be my favorite now.