12 ga.shotshells are an excellent defensive round at close range, whether loaded with salt, birdshot, buckshot, or a slug.
A gunfight is no picnic, and showing a gun should be absolutely a last resort, as when you face certain death or worse. The big problem is that if gunplay starts, you'd better be able to sustain the requisite amount of fire to settle things in your favor. (Probably nothing so adversely affects the good humor of bad guys as firing a round their way.) That says a repeating gun and a fair quantity of ammo, same as Captain Slocum. You also need enough practice to be proficient.
If t'were me on that cruise to out of the way places, I'd choose a stainless "defender" shotgun.
I would NOT load a shotshell in any 12 ga flare gun, at least any I've seen.