Barrels out of Bondage: Open or Sealed up Tight?

[Valandil]

Just saw the movie snippet again yesterday from "Desolation of Smaug" - where the dwarves escape from the Elves in the barrels. And of course - they're not packed into tightly sealed barrels, as in the books - but just take one barrel apiece, leaving one end open.

In the movie of course - this makes it more practical for carrying on a seemingly endless battle with the Orcs as they go down the river.

But which seems a more reasonable way to ride a barrel? In the book, the dwarves are packed in for a couple days (I think). It seems like drowning would be a very real possibility. Not just a near miss as a few of them had. I wonder if hypothermia would be a thing for dwarves.