Hi, just back to Pamplona after crossing from SJPP - twice with groups in the last two weeks.
Back to the questions of water. From SJPP, the first fountain you come to is at Gite Hunto (5 kms), and the second is at Refuge Orisson (8 kms). Both are outside and available to all as you pass. The gites do not have to be open for you to get water. Then just as you are about to cross into Spain, there is the Fountaine du Roland, which was running both times (and always has been on my previous crossings).
As for how hard it is, this depends on several factors. If you are not in great physical shape, you can expect the climb to be challenging. If you are carrying too much weight in your backpack, you can expect to have a difficult time, and you run the risk of injury which will affect the rest of your Camino. The weather can also complicate things - 30 degrees in direct sunshine, high winds, cold clouds, rain, etc., all add to the difficulty. You just never know what the weather will be on the mountain.
The other consideration that most people miss is that the walk from Roncesvalles to Zubiri (22 kms) can also be quite challenging due to the extensive downhill stretches. After coming down into Roncesvalles, the legs are a bit wobbly, and the knees hurt a bit, if you are not accustomed to walking downhill. That can make those next 22 kms quite difficult.
My conclusion is that each person is responsible for making the right decision for him and herself - whether to cross in one or two days, that's up to you. Take all the information available here, then make the right choice for you.