You won't find anything written because there isn't anything written. And that's a good thing.
The way our legal system is structured, if its not prohibited, then its allowed.
Laws don't grant rights, they restrict them.
That means there are no restrictions with respect to carrying on a boat. Of course all laws also still apply.
So if you leave your boat unattended, storage laws apply. (at least in MA)
But other than that, there's nothing.
Don
p.s. Some may want to cite this law restricting possession of a loaded long gun in a motor vehicle.
https://malegislature.gov/Laws/GeneralLaws/PartI/TitleXX/Chapter140/Section131c
one thing to keep in mind is that the laws use the term vessel not vehicle, when describing a boat.
But even if you take the absolute most conservative position, this law restricts the possession of a loaded firearm in a vehicle. So the firearm is fine, a loaded mag is fine, you just can't load it until you need it.
And of course, none of this applies to handguns.