Variation in water desity is a secondary effect
A 777 flaperon is constructed with upper and lower aerodynamic surfaces that meet at a sealed training edge. This hollow structure is vented in order to allow internal pressure equalization with the ambient during flight. Whether it floats or sinks initially will depend on how much of that internal volume of air escapes and is replaced by sea water.
If the flaperon looses enough of its trapped air to start sinking, the volume of the remaining trapped air will be halved when depth reaches 10 meters and halved again (down to 1/4 that on the surface) when depth reaches 30 meters. Surely this rapid change in trapped air volume will have a much higher impact on buoyancy than variations in water density that will be only a fraction of a percent.
There are only four states that the flaperon can be in at any point in time with regard to floating/sinking:
1. Floating on the surface with sufficient trapped air/gas to provide positive buoyancy.
2. Sinking toward the bottom at an increasing rate as it buoyancy decreases due compression of trapped air/gas resulting from increasing water pressure.
3. Resting on the bottom with negative buoyancy thus not able to float.
4. Rising to the surface with increasing rate as its buoyancy increases due to expansion of trapped air/gas resulting from decreasing water pressure.
Note that none of these four involve being suspended at a finite depth below the surface, yet above the bottom for any period of time.
Now consider changes that could occur to move from one state to another.
State 1 (floating) to State 2 (sinking) could occur from:
- loss of trapped air/gas due to wave action.
- loss of trapped air/gas due to venting enabled by damage or corrosion.
- loss of trapped air/gas due it being dissolved into the sea water.
State 3 (on the bottom) to State 4 (rising) could occur from:
- accumulation of trapped gas due to a chemical or biological process.
- detachment of a particularly dense portion of the flaperon due to corrosion rendering the buoyancy of the remaining system sufficient to initiate floating.
As for the Frechman and his 777 flaperon in a swimming pool, he could show that it sinks to the bottom or floats on the surface depending on how much of the interior volume he floods. There is no way he could end up with it fully submerged, half way down, neither rising or sinking further.