I'm by no means an expert on Japanese. Since no other person seems to want to answer, I'll give it my best shot:
Adding "の" to a verb nominalizes, or make into a noun, the verb. So "泳ぐの" is "swimming". "には" would be "In regard to" or "as for". "危険である" is "to be danger" So "This river, in regard to swimming, is dangerous." or "Swimming in this river is dangerous."
A better translation would probably "This river is dangerous for swimming." I forgot that "には" is commonly "for", which is much more smooth in English.
Another way to translate it.
"It's dangerous to swim in this river"
I don't really know what is the best translation between the ones proposed by KenpachiNoPein and mine.
I guess it's a subtlety I don't understand yet. Maybe something like the question or theme behind the sentence.
"Is this river dangerous?", "What is dangerous?", ...
Furthermore, I am not a native english speaker.
Also, a bonus example using the same pattern.
https://www.youtube.com/watch?v=CmgHKWXeiew
0:26
まだ泣くのには早いよね。
It's too early to cry.
But I didn't know we could help people on this website, I'll try to help people from now on.
Question about the uses of the particles ''のには''
Until ''泳ぐの'' I was understanding the sentence but after I become a little confused.
I don't understand how and why the uses of ”のには” to glue the adjective "危険", can someone help me?