No, you can't say that – you can never have a definite noun as "owned" by another noun in the genitive. You could say Sjöns vatten instead, but Vattnet i sjön is used more often, it sounds a little less formal.
Oh, it makes sense, because if you have a genitive it's obivous that the noun is technically determinate.
'The water is cold in the lake' was not accepted? Isn't that the normal way to say it in English? I'm not native English, so this is twice as hard.
As an English native, your sentence doesn't sound wrong to me, but it doesn't seem like the right way to express this sentence. Your sentence makes it sound like the water is cold in the lake is a temporary thing, as in the water is cold while in the lake. Also, in your sentence cold is only describing [the water]. You would want to say The water in the lake is cold, as cold is describing [the water in the lake], not just [the water]
I also wrote "the water is cold in the lake," and fully disagree with the other comment insinuating it is fundamentally different from, "the water in the lake is cold." Similarly, "in the lake, the water is cold" would mean the exact same as the first two.
I disagree. The phrase [the water in the lake] needs to be kept whole in order to retain the meaning
Thank you for your comment! The order of the words in the Swedish sentence sounds natural in Swedish spoken language. Or is it? Can someone native Swedish give a comment? And similarly, I was typing the order of the words in the English sentence as it would sound normal for me.
In Swedish, both Vattnet i sjön är kallt and Vattnet är kallt i sjön sound good, and I don't feel a big difference between them. I've also added The water is cold in the lake as an accepted answer because I think it'll in most cases be interpreted as the same thing.
It's complicated to explain, but it's the difference between "cold" describing "the water in the lake" or "cold" describing "the water" and "the lake" indicating position. It's nuisanced and very hard to actually explain.
I hear "pfön"... how actually to pronounce this "sj" combination. I failed in voice task
That's actually un-intended... we accept "The lake('s) water is cold", so Duolingo thinks the missing hyphen from "lake-water" is just a typo and accepts it. Generally speaking, Swedish loves compound nouns, so "the lake-water" would be sjövattnet. Since compound nouns are such a huge part of Swedish, we try to keep the constructions separated for the most part.