I get a similar effect by simply partially inflating the collapsible water carrier and canteens when and if it seems possible we will be around significant amounts of water. I have used them as floats to assist crossing water two or three times. Works great in the summer heat.

Even without added buoyancy I have found that because I use a lot of baggies and water resistant stuff sacks when packing that my pack tends to float fairly well. Canoeing in the summer with with camping gear, where a daily dunking is half the fun, our packs all tended to float pretty high. In fact so high that they tended to blow/float away. This prompted us to rig dummy cords on them. On the other hand the camp stove and griddle, understandably, sank like a rock.