During a 4 day trek in the Guadalupe Wilderness in west Texas I, like you, had to carry all my water for the trip. 10 liter. I am using a SMD Essences Pack which I have fitted with carbon fiber rods for better load transfer. My base pack wt is about 4.5 kg so I had to haul nearly 15 kg up nearly 1000 meters of elevation gain. So much for UL I thought, but then it occurred to me that my old (pre UL) backpacking base weighed was close to 18 kg without the water, so I was actually 3 kg lighter than I would have been even with the water. That thought didn't make the load any lighter, but I felt better about it.
I think even SUL advocates can't get around the water issue if they have to carry it, then they need a pack that can take the load or some how do without. And doing without might be dangerous in some situations.