In real life, mines (at their most basic, small containers filled with explosive, fitted with some sort of triggering mechanism, and left to blow up the unwary) are a mainstay of warfare both on land and at sea:
- Land mines are small, being mainly designed to blow off a soldier’s or animal’s foot or wreck the wheels or tracks of a vehicle (even the large kinds designed to disable heavy armoured vehicles are still fairly small), and usually pressure- or tripwire-activated (i.e., you step on [or drive over] the mine or break its tripwire, mine blows up).
- Sea mines are much bigger than land mines, being more-or-less exclusively designed for disabling or sinking large ships, and, while primitive types are contact-activated (where you have to actually hit the mine for it to blow up), most use some sort of remote-detection mechanism (where even sailing near the mine is enough to make it blow up).
There is, in real life, essentially no reason to develop naval anti-personnel mines, since humans are creatures of the land, and, even when they fight at sea, they do so with dry feet, from big vehicles that are essentially impervious to anyone in the water - actual fighting in the water itself is quite rare.
Mermaids, on the other hand, are creatures of the water, who live and fight and die in it, and would have very good reason to develop and use underwater anti-personnel mines, if at all possible.
I can already see a number of problems, though:
- Whereas human soldiers and their vessels are both mostly confined to an essentially two-dimensional battlefield (moving along the surface of the ground and water), mermaids would have the entire three dimensions of the water column to work with (at least until they get far enough down that they have trouble seeing in the dimming light and/or the increasing water pressure starts causing problems); thus, mines planted on the seabed (akin to human land mines) or floating at or just below the surface (like most human sea mines) would be mostly useless. Instead, the mines would have to be distributed throughout the mermaid-useable depth of the water column, which would:
- Greatly increase the cost and effort of laying a minefield;
- Most likely require that the mines be tethered to anchors on the seabed (for positive-buoyancy mines) or to floating buoys (for negative-buoyancy mines), in order for the mines to maintain their proper vertical position; as mermaids, unlike human sailors, would be viewing the minefield from within the water itself (without any of the mine-obscuring effects of looking through the air-water interface), this would tend to make the minefield immediately visually obvious.1
- The aforementioned excessive-visibility problem could be mitigated by hiding the mines in something like a dense kelp forest (assuming that you have a conveniently-located kelp forest or whatnot), but this would bring up yet another problem. One would presumably want to use some sort of contact fuse (probably pressure-sensitive plates or an underwater tripwire), since, unless your mermaids have very advanced electronics at their disposal, most remote-detection detonation systems would be unable to distinguish mermaids from the sea itself (mermaids - assuming that they have a remotely similar biochemical makeup to real-life animals - would have essentially the same magnetic permeability as seawater itself, making them invisible to magnetically-triggered mines, while the sounds of swimming mermaids would likely be very similar to those produced by ocean currents, waves, and other sea life, probably making acoustic mines useless). Unfortunately, contact fuses do not tend to be very discriminating as to what is contacting them (as long as it exerts enough force to trigger the fuse), and would render the mines highly susceptible to being set off by objects moving with the currents. This is not a problem for human land mines (as, even in parts of the world where the ground is prone to moving suddenly, it does not do so particularly often, and the air, while almost constantly in motion, has such a low density that it would take a very powerful storm to move objects heavy enough to set off a land mine), nor for human sea mines (as these are designed to damage or sink large vessels, and, therefore - where they even still use contact fuses at all - require far more applied force than any but the very largest current-borne objects are capable of exerting), but would be quite problematic indeed for mermaids using underwater anti-personnel mines, as a contact fuse sensitive enough to detect a mermaid brushing against it would likely also be sensitive enough to be triggered by current-borne fronds from the kelp forest hiding it.
- The much lower compressibility of water, as compared to air, would greatly increase the effective range of the shock wave from an underwater mine detonation over that from a detonation of the same size on land; on the plus side, this would greatly expand the mine’s lethal zone, but, on the minus side, it would require that the mines be spaced much further apart than they would be on land, in order to keep a detonating mine from setting off the rest of the field in a progressive sympathetic detonation (and thereby clearing the entire minefield in the process), potentially leaving enough space between the mines for mermaids to swim through without touching the mines at all.
- Finally, the much higher density and viscosity of water would render fragmentation (a major injury-and-death-causing-and-aggravating factor in land-mine explosions) ineffective as a lethal or injurious mechanism for underwater mines, which would, thus, have to rely entirely on blast injury; the fragments from the mine casing would be slowed down to mere nuisance speeds by hydrodynamic drag within a meter or two of travel, a distance well inside the lethal blast radius for such an explosion.
Given all this, would submerged anti-personnel mines be useful or practical in warfare between mermaids?
1: This would be even worse for mines tethered to floating buoys, as the buoys themselves would likely have to be tethered to the seabed in order to keep them from drifting offsite; thus, you would have an anchor chain or cable extending the full depth of the water column from the anchor all the way up to the floating buoy, plus another chain or cable connecting the buoy to the mine.