Guns just ruined this for me.

"Pirates," whom are not even actually pirates, wielding guns against... merfolk. And why'd you say "Swords and guns"? Swords were around before longbows or crossbows, and became more and more irrelevant as ranged weapons increased in number and power. Still useful, but not as effective. Combine that with guns... no reason for swords. A handgun from WWII, when magazines really started showing up, would take down a swordsman no problem. 

I could have a guy that carries a trench gun and an M1 Garand, and I'd be unstoppable as a soldier. Trench gun eliminates anything close, M1 Garand eliminates anything farther than that. And yes, one man can carry two guns + ammunition, I would know. 

Guns and mermaids don't fit. I think I'm gonna drop out because that just... doesn't make any sense. This mad dictator of yours would be dead or not nearly so powerful if guns truly existed.