Oyster tongs.

Probably not real availble up in that part of the world.... so what I would do is use a boat hook and try to snag it by the trigger guard or by the sling (if you had the forethought to attach a sling..... thats why ALL duck guns should have slings.... while ungainly and often in the way... it might help you get your gun back).

One of those long expandable decoy retreivers with the hook on the end would be perfect.

If you were in my part of the world.... it would already be gone... either rusted to death or the tide and mud would have silted it under.