So perhaps a better question would be has anyone tested to see how quickly the plane would have sunk?
A Lockheed 10E that ditched off Cape Cod in 1967 floated for eight minutes, but it didn't have the large fuel tanks Earhart's airplane had. How long NR16020 would float would depend on ow intact it was when it went over the reef edge. The tanks had drain valves on the bell and vents in the filler ports, so if scraping along the reef on its belly compromised the drain valves the tanks would fill quickly. When the plane slid on its belly in the Luke Field accident it came to stop in a big puddle of gasoline that, fortunately, was quickly washed down by the fire truck that was following the plane. Bob Brandenburg calculated that with just the two 102 gallon wing tanks compromised the plane would float for 18 minutes.
To wash the airplane into the ocean there had to be a lot of water running across the reef, so it seems safe to assume there was heavy surf at the reef edge when it happened. Anything that goes over the edge under those conditions does not peacefully float away. Even on a calm day, a diver in the water at the reef edge gets slammed against the coral. Having seen that environment first-hand many times, it's hard for me to imagine that airplane not sinking almost immediately in the relatively shallow water near the reef edge.