Re: Guns on boats. Marine laws vs state laws
To answer the question about what is "legal" you will find many overlapping rules and regs. Some are driven by your location: state + county/city + specific water (state park, army corps, etc.). Some are driven by type of gun and how it's carried (loaded, concealed, cased, handgun v long gun, within reach) and some there are exceptions if you are hunting as opposed to passing through.
In a "conversation" with some game wardens once, a guy I was with was in a 14' boat and had an old pistol in a shell box. Because the boat was so small, it was "within reach" and it was in the box so it was "concealed" and it wasn't a hunting gun, so he could have been cited for an illegal concealed weapon. And had it been the same gun in the same box in a 21' boat, it would not have been. That's one of the reasons I got a concealed carry permit, so I wouldn't have to worry about details, debatable standards and over-zealous LEO's.
Your hunting guns should always be unloaded when in a boat that is under power, because (a) that's basic safety and (b) it's illegal to shoot from a moving boat.* And the better practice is to have them in the case, too. There are very few instances where you can get in trouble for having an unloaded gun in a case.
Many places that ban guns make an exception for one while in transit (again, cased amd unloaded). But what about waterways near cities like DC and Boston that ban possession of many guns, including handguns?
I use boats for transportation to and from an island hunting/fishing camp, where we also target shoot. So I am always carrying guns on the water even when not hunting. As far as purpose goes, my boat on the creek is the same as my car on the highway, but different rules can apply. I've had that "conversation" too with wardens about carrying lead shot, not hunting but during the season. Anotehr $700 question.
*There is a conflict between Federal and Virginia law as to shooting a cripple (duck/goose) while under power so if you do it, you have to know which warden is watching, whether you are OK or will be writing a $700 check.