If you are in us territory carry whatever is allowed by the State who's waters you are in. It's when you are in foreign ports that you have to be careful. Don't carry a weapon so nice that you would hesitate to dump it over the side if necessary. That's why weapons that can be made of components that by themselves are legal would be useful. barrel and chamber liners can turn a flare gun into a twelve gauge pistol but can be removed and disguised as deck hardware. flare rounds can be reloaded to fire shot but would appear as a regular flare round. A crossbow can be made of components that when disassembled would look just like a assortment of spare parts normally found on a boat. As mentioned before a flame thrower can be built of things normally found on a boat. Scuba tanks, propane bottles, high pressure hoses, gasoline, oil, soap, valves all are not out of place on a boat.
Then there is the full size inflatable mock-up of a Los Angeles class attack submarine conning tower that you can inflate upon approach of a suspicious vessel.
Then there is the full size inflatable mock-up of a Los Angeles class attack submarine conning tower that you can inflate upon approach of a suspicious vessel.