That's what I would do... The second trip for the water should be fast and easy.

When I had to bring that much to drink I've used a 2.5 gallon flexible plastic container with a spout like this:



They pack well, seal pretty good, and are very durable. The spout is really handy when the container is set on a table or rock. I suspect that if you left a just a little slack in the plastic they would freeze just fine.

These actually do work really good for beer too. You have to get your beer really cold to lock in the CO2, and pour it in the container very slow so it doesn't foam much, and you have to fill it all the way up or get all the air out, no foam in the container, no air space. When you draw a beer the bag collapses so no air gets in it. Beer will stay fresh and carbonated for a couple days in one of these. Longer if you don't open the valve.

You might think the hardest part about using those for beer is hiding them in your buddies packs without them knowing, but it turns out to be getting them out without them knowing. I still haven't figured out how to do that...