This cylinder would displace 100 liters of water. To keep it vertical you would have to find some way to keep its center of gravity below the water line otherwise it will seek a position of lowest energy on its side. This would probably involve adding more weight as ballast to one end of the cylinder. As it is stated the 100kg would require 1/4 of the volume of the cylinder to displace enough water to float it so you have adequate volume to support near 400 kg.
Another thought. If you can hang the ballast outside the cylinder like a canon ball on a chain from the bottom of the cylinder this will do the best job of lowering the center of gravity.
As John indicates, if you have a rod rigidly attached to the bottom of the drum with a weight on the end of the rod equal to the weight of the empty drum, it will float high and vertical, even in waves.
