A PC needs one gallon of water per day. He can take half that amount but risks an exhaustion level if he fails a con save. Any less water is an automatic level of exhaustion.
The equipment chapter states that rations are just dried food, jerky and nuts and the like, not liquids. The water skin container can hold exactly half a gallon, not enough for one day.
If players can't really forage for food and water in a particular site, are they expected to carry one gallon of water per day? That's a lot of weight! I find it hard to maintain a sense of realism by picturing anyone carrying so much water while remaining able to fight, sneak, and fill his pockets with loot.
Best Answer
In short: yes.
In actual wilderness survival, having sufficient water is a pretty big deal. Even just to go on a long hike IRL can leave a large proportion of your carried weight as water. Any time I've gone camping out in the woods for an extended period, the weight of the water I had to transport was significantly greater than the weight of everything else I was carrying. Which is why I forage and purify water whenever I can, rather than trying to pack it all out on my back.
As you noted, the PHB specifies that a PC needs a gallon of water per day. Thus...they're going to need a way to transport one gallon of water for every day they are on the road. You are absolutely right that this is a lot of weight.
However, D&D does provide you a few options here, without having to consider magical items or spells. The simplest solution is a donkey with a pack saddle and a barrel. A barrel of water can contain 40 gallons. A waterskin can hold half a gallon and weighs 5 lbs. Based of the actual IRL weight of water (8lbs per gallon) we can assume the skin weighs one pound. Thus, a full barrel of 40 gallons of water can be estimated in at 320 lbs.
A donkey can carry 420lbs of cargo. So, if you need to make a long trip, you strap a barrel full of water to the back of a donkey, and now you have 40 gallons of water; enough water for a party of 4 to last 10 days without foraging. This only sets the party back 15gp (2gp for the barrel, 8gp for the donkey, 5gp for the pack saddle), and water is basically free as you either pulled it from a well or a nice clean stream. And any chance you get to forage, you just refill the barrels.
If you need more than a tenday worth of water, buy a cart. Per the PHB, an animal can pull 5x its carry weight if that weight is in a cart. This takes the donkey up to 2,100lbs of carry capacity, or 6 barrels of water (60 days worth of water). Again, this is cheap. A cart is only 15gp.
Naturally, if you have a donkey with you, it's going to need to drink as well. However, per the DMG p.111, a Medium Creature only needs one gallon of water per day. This is not exactly realistic relative to how much water a donkey actually drinks in a given day, but that is the simplification granted by the rules as written. So, yes...you are adding an extra consumer to the mix...but it can carry far more water than a hireling could, for the same water intake.
In short, if you are traveling cross country in the wilderness and are unable to resupply...do exactly what people used to do IRL. Bring animals along to carry it for you.
I would note, however, that finding a place with absolutely no potable (or purifiable) water would require them to either be in the deep desert with no oases anywhere nearby, or in a volcanic wasteland where all the water was tainted. In general...most areas, you can forage to resupply.