“Out they went under the overhanging branches of the trees on either bank. Bilbo wondered what the dwarves were feeling and whether a lot of water was getting into their tubs. Some of those that bobbed along by him in the gloom seemed pretty low in the water, and he guessed that these had dwarves inside.”
– Chapter 9: Barrels Out of Bond, The Hobbit
Atop his barrel somehow and managing a peek backwards, I rather imagined this is what greeted Bilbo’s eyes.