Bears have 1000 times better sense of smell than humans -- If you have seen diagrams of their noses vs humans' you can see why.
So if you hide water in the trunk vs food, they can smell the difference. They will leave the water alone. If you are really in doubt, just put the water container under the car or right at its front bumper -- somewhere where people will know it belongs to you. The animals will leave it alone.
...but I think there is a water supply at HM, so a large container wouldn't be necessary. I hope someone can confirm this.
