So since SOTE doesn't have a canon semi-aquatic race, I decided to try and come up with one better than the kappa half-turtle memes. I based these guys as a humanoid version of various species of sea turtles, I did a bunch of research but if anyone knows more info about turtles or sea biology that might affect how these guys operate, feel free to give feedback.
Shellfolk would be quite tall at roughly 6-7 feet in height, and in place of flippers, they would have long limbs, possibly with webbed feet and hands(?). They would be able to hold their breath for many hours (some sea turtles can go without air for close to half a day, although I'd imagine a larger creature would need oxygen more frequently).
They would be omnivores and mainly subsist off of seagrasses and seaweed, as well as both shallow water and deep sea fish, crustaceans, jellyfish, cephalopods, etc.
They would be highly k-selected and nomadic, have a lifespan of hundreds upon hundreds of years, and would mature extremely slowly. Nomadic shellfolk would seasonally migrate along foraging and nesting sites, sometimes across thousands of miles between oceans.
Shellfolk would be capable of permanently settling on land, but would also require near-constant access to a body of water or they would die of dehydration over the course of several days. They would be stronger than a human, however they would obviously be very slow on land, making them poor raiders (except as pirates possibly).
Shellfolk would likely skew feral due to their small population and difficulties of living on land, though they’d be capable of forming long pockets of civilization along the coastlines of islands, major bays and rivers, wetlands, areas with seasonal monsoons, etc, with the largest of shellfolk cities being based around carefully managed and farmed estuaries. They could also form extremely defensible settlements on islands in the middle of lakes, similarly to Tenochtitlan in our world.
Terrestrial races could even employ shellfolk tribes as vassals to guard vital rivers or straits.
Assuming their shell is modeled after something like the above (covering both the back as well as the front), it would make shellfolk virtually immune to most attacks from behind or at the midsection, protecting them from predators and raiders. Their main weak points would be their neck, limbs, and head, areas commonly covered in armor.
If forced into conflict, almost nothing would be able to overcome a formation of armored turtlemen defending an amphibious position (pre-gunpowder weapons). However, due to their generally peaceful attitude towards other races, along with their small population, stable politics, and long lifespan, shellfolk would often have a reputation as wise old pacifists.
If kappas stick around, they could have a pretty interesting relationship with shellfolk (presumably r-selected vs k-selected, entirely feral vs somewhat feral, raiders vs non-raiders, etc), but that's getting out of the scope of this suggestion and I probably already went too long.
Let me know if you have anything to add.