Mariner:

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.

Scott

Land: I’ve spent a lot of time whitewater rafting and kayaking and, in my experience, the only dry bags that are truly dry are Watershed bags which have a zip-lock type closure but are pretty expensive. Most roll-top style bags will leak a bit through the top roll but if you roll it correctly the leakage should be minimal. If you have anything else in your pack that is critical to stay dry (e.g. phone, key fob) you should probably put it in a zip-lock sandwich bag just to be safe.