My novel taking place in an early 1800s setting (circa 1800 - 1850) has a human versus merfolk war. It started with disputes over fishing resources, but now has escalated, for the merfolk, to a fight for survival, as their empire over the ocean is collapsing, and they need to raid human shipping just to survive.
The merfolk can attack human ships by cutting into the hulls and sinking them, or using grappling hooks to pull attackers into the water. Humans can attack them with firearms or melee weapons if they get too close.
Problem is, neither side can directly affect each other - until the humans, desperate for the "impossible war" to end, spearhead research into what ultimately becomes the depth charge. Using this, without warning, they launch a coordinated barrage against the merfolk capital city, destroying the empire, and fragmenting the merfolk into tribes and city states.
Now, all that out of the way, is it realistic for depth charges to exist with 1800s technology? Could they detonate just by contacting the seafloor? Would they need to detonate via some kind of fuse? What difficulty would the humans have in acquiring enough of these to level a city? What kind of supporting technologies would be needed for this?
Answers to any of these sub-questions are much appreciated.