'Para' can be to or for so my answer of "Water is important to humans." should also be correct, right? Or, am I missing something?
"Water is important to humans" sounds perfectly fine to me and I think it's a perfect translation for the Portuguese sentence. Unless there's something subtle between "for humans" and "to humans" that I'm not picking up because I'm not native.
I think "for" suggests something more practical, and "to" suggests something emotional, but they're practically the same. Your respect is important to me. Food is necessary for me to live.