indeed. Funny enough, only the non-winched down balloons in multiplayer seem to have these rigid properties a rudimentary buoyancy simulation. The winched down ones (like in the singleplayercampaigns) simply explode on the slightest bit of collision detection. When you drivem over with a floatplane (the idea beeing that the floats protect your prop so you can infintifly do that and get home), you might even get the kill for that.
the last example with the floatplane at 1:20 in this old vid was for demonstrating that:
Keep going,I'm sure you will do it.