The question isn't really hypothetical because, inevitably, someone here will find a weapon while diving. This year alone we've found quite a lot of unspent 30-06 ammunition from the WWI era, and a CO2 air pistol (badly corroded).
Let's assume that you find a handgun while diving.
Common sense says you shouldn't play with the thing and probably leave it alone. At the same time, however, it might have been used in a crime and should be returned to the police. If you leave it, it may be difficult to find again. If you take it, you should at least make sure it's unloaded before toting it anywhere. I suppose the answer may depend on its condition and your own comfort level with firearms.
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