Ric said, above; "...Sheets of aluminum don't float worth a darn so in order for the piece to wash ashore anywhere it must have ended up in water shallow enough for wave action to move it - no deeper than about 50 feet would be a good guess. Another indication that it was once in relatively shallow water are the spots of coral growth on the surface of the artifact. Coral only grows in sunlit water."
Here is a bunch of posts in a completely different aircraft forum that shows examples of crashed WWII aircraft parts that washed up onto beaches. The ocean has the ability to move objects from deep water up onto the shore, and relatively light objects with large areas for currents to act upon, such as sheets of aluminum, are more likely to appear than dense heavy objects, although there are examples of those, too.
The point Ric makes about coral growth is a clear indication that the object definitely spent some time at shallow depth, but it also might have spent some previous time at some greater depth.
An entertaining and related book about Flotsam, Jetsam and Lagan ("Goods or wreckage lying on the bottom of the sea") is "Washed Up: the curious journeys of Flotsam and Jetsam" (S. Moody, 2006)