Despite what there location may suggest, they are not cold deserts like Antartica.
While they are not connected to any other continent through land, they somehow lack the cold circumpolar currents that make Antartica the wasteland it is. While the temperature is at below freezing point during particularly cold nights and way below in winter, it is much more bearable. There normal average temperature is around 10°C in summer during day but sharp decline at night, making it much more habitable with some forest and grassland in their coastal areas that have adapted to lack of sunlight for six months and permanent snow sheets covering the center.
Also, due to participating in hundred year war for so long and constant raids, there gender ratio has become skewed a lot in favour of women, as men has died a lot, in wars, hunting accident, fishing accident and what not. Alcohol, which this world has as I mentioned I plan to make it realistic, and smoking, while limited because lack of availability, high cost and much more healthier lifestyle, has also contributed to this. Same is for Northern Tribe but not to such an extent. So don't bother if I make polygamy normal in Water tribes. They need to repopulate, it is necessary.
And even if it makes it look like I am objectifying women, tell me what is objectifying more, not taking relations seriously because there is an abundance of women for you or managing multiple relation. I personally think it's the first.
Last thing, atleast for now, Tui and La don't remain on Human Realm all the time, instead like Netfix show comes to Human Realm ones every year on full moon. Mostly because Zhao found in spirit library that killing moon spirit will destroy the source of water benders power. If even one of Zhao's men knew his plans, they could spread it and when another imperialist Fire Lord or maybe even Earth King comes, they would target the moon spirit. It is much easier to arrange protection for them once a year than whole year round. It is specially for cases like Sozin comet.
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Author's Note
This is not canon but I am making it so.