The world-building community released a prompt about underwater engineering, and this is something I've wanted to dive into... get it? - for a while now.
Here's A Link To The Prompt If You'd Like To Check It Out.
The Kranii
There is a world on the fringes of the known galaxy known as Wag'wua - which roughly translates into "Water World" and it consists of over 90%, and the land that there is, mostly consists of dense jungle, or soggy marshes. There is no continent on this world, and instead, the entire mass of land is divided up into small islands, the largest of which is home to a very small port, which gives access to the land below the waves: Sou'intisqia.
The Kranii themselves never developed to live on land indefinatly, and after a small amount of time outside of the water they need to rehydrate. For longer voyages outside, they need Aqua-suits - specialised outfits that contain a small amount of water, and a large filtration system on their back, which ensures that the water can remain clean and breathable for long periods. The suit's filtration system and the water can be heavy, so it takes Kranii a lot of training before they can be trusted to leave in this specialised attire.
Mostly, the Kranii are peaceful, and they are one of the only races that weren't affected during the occupation. Their planet wasn't worth taking over, considering the small port, however, in some small bouts of conflict their islands were damaged destroying a portion of the tree life on their world, which has resulted in many people noticing a slight lack of oxygen.
"Breathing is noticeable when down there; uncomfortable, like you're a breath short of a full lung. If you're visiting, bring a canister of oxygen, or two, just in case." - Captain Suntari of the TSV-93 (Rock Hopper)
Sou'intisqia
The city itself is massive and has been added to for centuries. Many races have embassies down there, and it's a mix of waterways and airways. All races are accommodated to down in this city, and there is a plethora of suits available to cater to each person's needs and allow for mixing in "Common areas."
The Kranii, while they consider this to be their capital city - are a very adventurous people, and each family has their own domed, ball-like structures capable of directional travel. Sometimes though, they will raise anchor, and drift with the currents when they want to relax and get away for a while.
They don't consider land under the sea to be theirs, so can anchor their homes anywhere they feel like, so they're not attached to one single part of their homeworld.
Illusive
They are very hard to come across on the surface world of any other planet, as they tend to only visit other systems for diplomatic reasons.
The Kranii have a strong military presence and have been used in multiple missions throughout Free Space, but they have a weak Navy, and their ship-tech leaves much to be desired. They tend to focus more on underwater vessels, which have been made useful by the wider Free Space government. However, underwater weaponry, ships, and equipment can only be so useful, so rather than war efforts, they have made a lot of developments and became popular for tourism and recreation activities.
End Notes
This is the first time giving a name, location, and world name to these people. I've had an idea to include a race like this, but the time just never came up.
I knew they wouldn't be a major part of the stories, or world, but I like the idea of having something like this crop up at some point.
I'd really like to try and come up with what they look like and model some tests with polymer clay just to have a visual guide for them when writing them.