So you're basically looking for something to keep the kit afloat? (Not a life jacket). As long as it is (almost) waterproof it should float... (for a while). Doesn't matter if it's heavy, because it probably is bulky as well, and as long as you're not exceeding one kilogram per liter it will float.

When canoing, I've used bags and rucksacks lined with garbage plastic bags to that effect. Clothes, tent and sleeping bag are quite bulky even when compressed. When soaked through clothes themselves will most likely stay slightly bouyant to neutral (bobbing just below the water surface), but if there are any additional heavy items (such as an axe) those items will drag your pack down. If dry, clothes will keep quite heavy things afloat. The plastic liner will keep the water out long enough for you to salvage that bag. If the plastic is new and/or robust it will be almost totally water proof (you may have some small leaks around the opening). If used for a few days you will have some punctures and some of your stuff will be wet, some of it dry and some of it somewhere in between.

So my take on this - you don't need extra flotation, just make it water proof!