The problem I see with a barrel is that it would be a pain if you need to pack it over land for any distance. What happens if you use it to cross one flooded area, then get to land and have to carry it, then have to cross another flooded area?

The dry duffle like I described above is a much more versatile solution, in my opinion. You can carry it on your back, take it off and float it, then easily put it back on your back.

How high either one floats will depend entirely on how full and heavy you pack it.