If the flaperon has areas which are twin-wall honeycomb construction, it could easily be buoyant. Each closed off honeycomb cell acts as an individual buoyancy chamber. The flaperon as a whole would be unlikely to sealed in any significant volumes as that would create pressure cycling and potential fatigue, in fact even honeycomb is sometimes punctured in manufacture to prevent pressurising, although they may still hold air when submerged on the ocean surface.
Here are the relative densities of materials which may have been used in its construction (water = 1, sea water = 1.025):
- Aluminium Alloy 2.7
- Carbon Fibre Reinforced Plastic 1.5
- Glass Reinforced Plastic 1.8