That is a very nice find. Definately a keeper. It looks to be a Browning M3 aircraft gun. That is a modified .50 cal Browning M2 fired remotely from a helicopter or fixed wing aircraft with a rate of fire at around 1150-1250 rounds per minute.
Aircraft mounted M3.
If you found it alone then the poor sap who lost it was probably shot down by the Japanese.
There should be some plane wreckage nearby I would think. Those were used on most US fighters and bombers and such. The only way I see that gun being out of a wing or turret is the bad way.
Finding a Ma Duece would be incredible! They are by far the most iconic firearm in the current US arsenal in my opinion. The coolest part being the current models are essentially the same as the one posted here. Not much difference at all.
Great design, highly effective and successful weapon. John Browning was a genius.
It might be come from the America WWII warplanes as B-17 or B-24 or other bomber. Sometime, they flew low with something wrong while flying, cause more weight, so all crews had to throw the guns, flak helmet, flak vest, gunner's bullet cases to the sea, that way, the plane can fly to land on the ground than crash into the sea with too heavy weight with not enough power engine problems.