I carry virtually everywhere it's legal, including while boating, and frequently camp, using one of my several small boats as transportation, and bring along at least a potent sidearm, usually a suppressed .22 pistol, and sometimes a rifle. Of course, waterfowl hunters, etc., carry guns in boats. I've never been in a suitable place, but recall reading about squirrel hunting while floating down some stream in canoes or similar small boats --- sounded way more fun than fishing...
Of course, you want to be careful about not losing guns overboard, or getting them wet (especially with salt water). For sidearms, consider using a lanyard (Gem-Tech, the suppressor manufacturer, offers a nice one), and for long guns, cases that offer some splash protection, and, can be easily secured to the boat, the Kolpin Gun Boot being an excellent example.
In small boats that might predictably capsize, thereby dunking the gun, consider carrying a gun that's pretty much impervious to water damage, and easily rinsed of fine grit from the water, and restored to service without tools, e.g., a Glock, or similar striker fired pistol. S&W revolvers, for example, are a PITA to salvage after dunking, and require at least a set of three screwdriver heads to dis/reassemble properly.
As for legalities, do your due diligence for the area of interest --- there maybe anomalies of overlapping jurisdictions, etc., to confuse the issues, but for Inland and Western Rivers, I think state regs prevail, with the exception of National Parks, National Seashores, etc., which now mostly are congruent with state laws...