Well after nearly six months I suppose I do have more complex thoughts on The War Between the Land and the Sea. Well, more like a specific thought with sub-thoughts.
The main thought is that the reveal in the final episode that Aquakind have been secretly melting the polar icecaps not only completely derails things thematically, it also provides a justification for the actions of the "kill all the fish people" conspiracy.
Let's have a look at the specific lines that make up this reveal (transcript source):
KATE: Granted. This is Professor Bello, Tactics and Planning. What have we got?
BELLO: You were right from day one.
SHIRLEY: They were hiding it. You said so. It's all been a distraction.
BELLO: The diplomatic talks were just war games while they were doing exactly what we thought they'd do. Aquakind are melting the ice caps. They were clever. We were searching with satellites, but we forgot how good they are with water. They positioned a layer of ice crystals at three kilometres to hide the heat.
KATE: How did you find it?
BELLO: We didn't. They dropped the layer. They want us to see.
The talks between humanity and Aquakind, which have been the focus of the previous episodes, are explicitly and directly stated to be a distraction. Aquakind began the talks while doing this. Then when the melting is brought up, Tide admits to it, with the rather childish justification that humanity started it. He does this as the official ambassador of Aquakind. His parting words are:
TIDE: And be certain of this, little people. This was never a war. This was an inconvenience. You were the dirt that gathered while we were away. But now we have returned, and you will be cleansed.
This very much has shades of the Daleks. It also means that from the beginning, Aquakind has not been negotiating in good faith. From humanity's perspective, Aquakind cannot be trusted. And they learn this at the same time Aquakind escalate to openly genocidal rhetoric.
Meanwhile, the conspiracy to kill the fish has been operating in secret. Given they have also been trying to sabotage the negotiations in various ways, this rather suggests they believe that humanity is otherwise going to accept whatever terms Aquakind present. It's very much "we need to Get Them to maintain our wealth and power". Except the icecap reveal means it switches gear to "we need to Get Them before They Get Us".
It also means humanity is no longer just fighting for itself, but essentially for every land-based species on the planet. If Aquakind melt the icecaps, it would cause massive ecological disruption and devastation across the planet (include to the oceans but that point never comes up). All their complaints about environmental damage are effectively rendered null, because they are willing to inflict it to win.
Aquakind see themselves as superior to humanity, and believe that gives them to right to make any demands they wish. They openly refer to humanity as "dirt" that will be "cleansed". This is their official position, as presented by their ambassador.
Then, whoops! It turns out humanity could fight back. Aquakind's sudden death by plague is tragic in the classical sense. In their hubris, they reveal themselves, beat their opponent into near-submission, only to then find themselves at the sharp end of the knife when their enemy chooses survival over honour. Humanity started melting the icecaps. But Aquakind started the war.
It's very much a repeat of the problems I see in Cold Blood. The writers clearly want to have a morally ambiguous conflict between the two sides. Except once again, the two sides are not the same. Humanity is portrayed as having a mix of views, even within the officially sanctioned side (and even the French army lady is vaguely aware doing Severance is immoral). Aquakind, meanwhile, are almost entirely aggressive, and Salt only really changes sides after she's been labelled a traitor.
Since it's also part of the RTD2 era, it comes with the particular dubious texture as on Doctor Who. The fact that Aquakind has officially escalated everything goes unmentioned. It's all Barclay's sad fish journey, into Aquakind dying badly. The show's official position is "wow the mean evil humans were mean and evil", which ignores that Aquakind were written as having equally murderous intent.
Then there's the "Accord" thing. It's completely ridiculous. At the point Salt reveals the idea, Aquakind have already destroyed their credibility. Additionally, Salt herself previously pulled off the evil genie thing in Plastic Apocalypse:
SALT: Do they want to accelerate this process?
BARCLAY: We are committed to undoing the damage we have inflicted on our planet.
SALT: How committed?
BARCLAY: It is our greatest desire.
SALT: Then we shall grant it.
BARCLAY: Sorry, what do you mean?
SALT: Your greatest desire... is happening.
Then all the plastic and such in all water all over the world flies out onto land. The "hee hee hoo hoo this is what you asked for" stuff isn't cute when a weird faerie king does it. It's even less cute when you do it as part of negotiating an agreement between two peoples. Aquakind, Salt included, repeatedly fuck around and act in bad faith. Only an idiot would think asking for accord would actually be useful at that point. Of course, UNIT are idiots, and can't stop taking Ls even in their own show.
I'm not sure what the conclusion is here. The show is structured in a way where the use of Severance is meant to be unprovoked. But, well. While the development of the virus certainly is, by the time it actually gets used, Aquakind have effectively declared that in five years they're going to kill everyone. Twice, technically, since they also did the whole "Ambassador Rust" thing that went nowhere.
The War Between the Land and the Sea really is just disjointed and badly written