use a floor with the keg
I put a keg in the dunnage load once and then, because I wasn't as quick as I am sober, ran the boat over a rock. The rock pushed up, the keg pushed down, and together they punched a hole through both sides of the floor, making for a real pain of an on-river repair. Ever since then I've been a huge fan of suspended floors in any raft I row.
Also, if you set the keg at the waters edge to keep it cool, tie it off! Although having most of us paralyzed while one hero dove into the eddy to save our beer was pretty priceless...