Barrels that hold liquid are watertight when the wood has absorbed the liquid,
but if allowed to dry become slack. Barrels that hold apples are not
watertight, and don't need to be. I doubt that the barrels are floated
upriver -- instead they are brought up river by boat, and float down
river empty. Laketown may even fill them with tradegoods, if they have
anything to trade with Dorwinion.
So if the dwarves are lucky they would neither sufficate nor sink. But it
is an extremely risky plan, not just because the dwarf-filled barrels are
obviously heavier than normal and therefore might attract attention, but also
because the leaky barrels could sink, if they are too dry and leaky, and
could become airtight, if they absorb enough water.
In short, I think it would be like floating in a covered, spinning boat with a
slow leak of water on the bottom, and air on the top, and hoping that the leak
is slow enough that Bilbo can get them out before they drown, and fast enough
that he can get them out before they suffocate. Frankly, if I had been
Thorin, I would have opened negotiations with the
Elvenking!
“I dislike Allegory - the conscious and intentional allegory - yet any attempt to explain the purport of myth or fairytale must use allegorical language. (And, of course, the more 'life' a story has the more readily will it be susceptible of allegorical interpretations: while the better a deliberate allegory is made the more nearly will it be acceptable just as a story.)” (From Tolkien Letter # 131.)
