The road is the geography
On the surface of the earth the hills, rivers and valleys are important to us mainly because we walk everywhere we go. The exact height and incline and the condition of the ground, anything that makes it easier or harder to get where we want.
If we were birds, how would we think about geography? Well, it would be pretty complex. They live their lives in close connection to the ground, they eat and sleep there and most of their threats come from there so they would certainly keep a close eye on what’s happening. They probably wouldn’t care about short but steep shits in inclination that can define human life. They would notice a hill but a fence, wall, or a fortress, would be nothing to them really.
What's really going to fuck up a birds day is the wind. If the wind is in the wrong position or going the wrong speed or at the wrong height or full of hail or whatever then that has a massive degree of control over where the bird can go. So the mind of the bird is split. It has a huge degree of choice over where it goes, and it needs it because its probably burning a huge amount of energy to do anything and is only ever a day away from starving to death in most cases, but that freedom is based on the condition of another element of as great complexity as the earth yet less predictable.
If we could move though space in our dreams and wake up on the other side of the world yet could never fully control exactly how those dreams would go then we might begin to understand a little of the life of the bird. Two worlds, fluxing, interlinked.
But the key point here is that shit on the ground matters because we are on the ground and it matters to birds because they also interact with the ground.
Now, do fish fucking land at any point?
Not really. A few live on the surface of the bottom of the sea. Most float and swim. Several interact with that surface. Almost none that I know of land on the bottom of the sea.
And in the sea, food doesn’t come from below but from above, or all around. The system of the sea is that generally stuff comes from above and is collected in larger and larger animals and at some point, if you are a fish, one of the will interact with you by either being eaten or trying to eat you.
We would have to say that as much as birds interact with the surface of the earth then fish must interact with the bottom of the ocean much less.
So we come to the maps of Mermen and Tritons. How much would these maps show the bottom of the ocean?
The answer is not that much. An earthbound fortress has much less utility if anyone can attack from the 'air'. Walls are almost meaningless. Hills and valleys have some effect. There are various complex and interesting things happening on the ocean floor that must be accounted for. Lava flows, rivers of ultra dense saline water etc.
It’s the other way round from on the surface, in the sea, civilization is a pattern of movement, poverty and rebellion are still because stillness is bad. If you are trapped in a still piece of ocean then nothing is brought to you and getting anywhere is as hard as it is going to get so pirates and the Orc equivalents of the sea will hang out in flat still central patches of sea where there is not much going on, the deserts of the oceanic world. They will raid from there because its always going to be difficult for an organised people to reliably project force outside its accustomed routes.
In addition, on land we would consider the nature of the soil and environment as of prime strategic significance because that decides how much life it can support and what your population density might be, but in the sea the ocean floor has very little effect on that, what matters is the salinity, density, temperature, light levels and any confluence of currents, along with the migratory habits of any animal life, these are the source of your crops and these will define your population base.
On a coast or in a reef this might be different but think how small these places are when compared to the great deeps of the ocean.
What really matters is the speed and direction of the current because more than anything that decides where you can and will go. In a strong North-heading current any attack moving north will be rapid but almost impossible to retreat from, any attack moving south will be very difficult and unexpected but super easy to retreat from if you need to.
Add to this that probably everything is moving. So what matters is not position but vector.
Forming a local, or global 'loop' in which a current feeds back on itself is of tactical and strategic value because if you control the whole thing then you can reinforce yourself from any point by simply feeding military force into the loop. In the same way, breaking or interrupting such a loop would be something wars were fought over.
A small coastal or in-ocean loop is a duchy or nation, a gigantic global one is an empire. The emperor of the Mermen has, not a palace, but a migration. Actually he probably has a migrating palace in or around a migrating city.
(The city will be modular and the high status parts of it will be towards the 'front' or head of the flow so everything they drop and discard will drift past the lesser areas, the slums are at the back and rubbish and bits of crap from the whole life of the city drifts past there in a continual stream.)
If two great interacting oceanic loops are in a state of conflict, then the area where they meet may be a chaotic 'border princes' area with tiny bubbling kingdoms carved out in the midst of disorder.
So if there are ruins, they might be built on the ocean floor, or moveable cities that have sunk, but they are probably still floating around somewhere, far far away from the civilised paths, or deep in the cold zones.
The maps of the mermen will have ocean floor features mainly as navigational aids, in the same way we use mountaintops, or stars, as means to make your position, not as places to go or the means to get there.
Probably, considering the difficulty of producing even basic 3d maps the oral culture of mapping will be much more powerful. The song lines will give linear routes exactly as they would for non-literate human cultures and the shifting intersections and layers would be given by changing ‘decision points’ in the structure of the song. Information about depth, salinity, light, current etc could be given by rhythmic and tonal variations and shifts in poetic construction, which can pack a huge amount into a very small cognitive space and still be remembered and transmitted. (So, for instance, a shift in oceanic depth for a route could be signalled by an actual shift in tonal depth in the song describing that route.)
And that is why mermen and mermaids and whales are singing all the fucking time, they are trying to work out where they are, running through the map of a complex shifting three dimensional environment in their heads.