Does look like a small seaplane pontoon - or one off the end of a wing on a larger plane like the Sikorsky S-42. Or, could be something off one of the smaller experimental models. There was a wartime experiment with Spitfires to make some into small seaplanes as opposed to the larger flying boats. Or the Grumman UF-1 Albatross. Or just a tank of some sort, like a spare fuel tank for long range flight!