From the introduction:
Imagine some far point in the future... 1000 years from now. A great disaster has either forced humanity to leave earth or drove it to extinction, one of the factors is the earth's sea levels somehow rose some 1000 meters from where they are today. The combination of this with increased seismic and volcanic activity from the increased pressure on the earth's crust has created a far different landscape today. One where seemingly endless oceans greet you. Dotted by islands of various sizes. But the worst has happened and passed. You live in a settlement that has just about all you need. Yeah, you still work every day, but compared to the hectic life of the “earless” before them, the Kemonomimi of the endless blue live for their friends, their communities, and for the almost daily adventure that can be found all around them. You play one of those enterprising Kemonomimi. You're a diver heading down to the scariest ruins of the past, or you and your friends are the best surfers in the region, or perhaps you and your brave team of explorers have set out on a long journey... anything is possible in the endless blue, and there is a lot of it!
The climate in one word, is warm. One could wear a bikini in most of the regions of the world. The water and air near the poles is cooler, though, even with almost a millennium of climate warming, where swimmers need drysuits to be warm when in the water. Almost no snow falls at all at sea level outside of the occasional mixed flake near the poles. There are no icebergs anymore, and snow is seen only at very high elevations. Most settlements are dotted along the equator, With many of the old mountain ranges providing other sites for settlements.
Sea life changed as well, strange mutagens that created the Kemonomimi also created many strange new life forms with crazy environmental adaptations. The Kemonomimi themselves aren't boring either. They can dive to amazing depths and not suffer the bends like Humanity did on surfacing (although it might still be painful to surface too fast, and diving too deep does start causing hallucinations in Kemonomimi if they're not careful), also Kemonomimi can survive drowning, able to stay unconscious in a hibernating state until they somehow get oxygen again. Also their ability to take punishment (no doubt from their ability to dive deep) is quite notable.
But life as a Kemonomimi is one of parties, exploring, and it seems at times showing off to their friends. And they wouldn't have it any other way.