I maybe leaning a tad to the extreme here...call me paranoid; call me prepared. As a fairly recent combat veteran, small arms expert and FL CCW carrier, I'm always armed with something. If you carry every day, you probably have a favorite weapon for that purpose. Just like with anything else, the tool should be suited to the purpose. Your daily carried pistol/revolver might not be the best for your cruising situation. It certainly is no good at all where your CCW is not reciprocated. First consider your age, health, physical agility and acuteness of your senses. How close is an intruder likely to get to you? Are you a sound sleeper? Do you have any other means of early warning? The worst case scenario is that someone gains access to your vessel in the middle of the night and you come face to face with the intruder in your quarters. That's close! Spread it out a bit and lets say you catch him in the salon. These are scenarios where a short shotgun is preferred. One shot, big mess, but no more threat. Sorry...I don't care where I am...in this situation, I'll mortgage everything and then some for a good attorney, reciprocity be damned. Here in Florida we have the "stand your ground" and "castle doctrine" which clearly justifies these actions in this type of scenario. Other states may have similar laws, but there maybe a law that gives you (the property owner/victim) the legal obligation to retreat before the use of deadly force. Forget that! I shoot first and hire an attorney second.
Offshore is another matter. Personally, I prefer something in .223/5.56mm with stainless and composite construction. I want something reliable with a little bit of range and accuracy that will stand up to the marine environment. I also want a weapon that I can accessorize with a "Surefire" LED light and a laser. A laser isn't always for firing accuracy! Its a pretty powerful deterrent once someone notices that they have a bull's eye on their chest or their partner's forehead. Let's face it, there are pirates out there and a slow trawler is probably a pretty valuable target to a marine criminal. I've read some stories about hijackings and such where pirates are more interested lately in fast boats that they can acquire, use to run drugs and ditch all in the same night. A trawler wouldn't likely serve that purpose, however if I was a criminal out looking to score some cash, jewelry or other valuables, I'd definitely hit a trawler at anchor or under power in the open. This is not a situation ideally suited to a pistol or even a shotgun. This is the type of scenario that you want to maintain some distance between you and an approaching vessel. Clearly your first defensive weapon will be the VHF, every light you have on board and someone behind you ready to launch a flare sortie. Generally speaking, once it is clear that a criminal's intention will be met with armed resistance, they'll go away.
Every boater has heard stories of piracy in the Caribbean and open waters of the Gulf of Mexico. Whether or not you chose to believe the stories is up to you. Preparing yourself for an encounter with marine criminals is also up to you. You might sail/cruise all corners of the globe and and never encounter someone intending you ill will. It could happen tomorrow that someone flags you down in "distress" in open water. I'm going to be prepared to defend myself, my family and my property in that situation and I make sure that it is clear that I am armed from a pretty good distance away.