That’s a unique sounding problem. I like to use a bunch of smaller containers so they can sort of conform to my back. So, conventional wide mouth liter water bottles would be quick to fill but your need a bunch of them. You could just wear running shoes and not worry about getting your boots wet. Water is not as dense as sand or rocks so the water volume can be an issue when the weight gets high. There’s nothing wrong with carrying the weight down again. Its actually much more demanding and more specific to climbing. If your legs are strong that could be your solution and avoid water all together. Make some sand bags.