Friends have been asking me “How did a metal chunk of an airplane float to Réunion? Why didn’t it sink?” Or, as Monty Python might have asked, “What also floats in water?”
I talked to a former Boeing engineer. It seems that the 777 flaperon is hollow, comprising ribs and an aluminum skin. The same riveting techniques used on the pressure vessel are used on on the flaperon and therefore everything is airtight by default and therefore water-tight. He didn’t remember if there were any drain holes in the flaperon, as there are for some control surfaces, but they would have been very small and easily clogged, thus trapping air.
Trivia: The 777 was originally designed with folding wingtips so that it could fit into gates designed for the B767. This was abandoned at some point but the idea is back with the 777X (gizmodo).