MrSnuggles,
Unless we know the density of a 777 flaperon, we can not say conclusively that the "drifting below the surface" scenario is wrong ...
Well, the notion of drifting below the surface, i.e. "neutral buoyancy" is a bit of a fallacy. Things float or they don't. What can happen is that temperature effects can cause objects to rise or fall, e.g. in a thermocline, but for 2,500 miles - no!
My examination of the B777 Flaperon construction ( Boeing is not helpful in this regard) tends to lean toward a familiar spar, rib, stringer construction but using CFRP materials. The unknown area is what is actually happening between the forward spar and the aft spar? What the construction was on the trailing edge is fairly obvious from the many photographs available. There is also an obvious drain hole located just forward of the aft spar on the
underside and outboard of the inboard rib - [if that makes sense].
There must be a way for any void compartments to "breathe", otherwise pressure differentials will cause damage.
My assumption is that the Flaperon floated topside down, with its center of buoyancy about 25% aft of the forward spar.