The only things afloat in The Hobbit are the barrels which carried Thorin and Company out of bond. It comes from “on float” with a reduction of the first element in the “a-” preposition1 sense: expressing a position within a situation, just as in “afire”.
- 09.052 so as to let out the barrels as soon as they were all afloat below.
- 10.009 in the meanwhile the barrels were left afloat
“afloat, adj. and adv.” OED Online. Oxford University Press, March 2015. Web. 18 May 2015.