Is the verb "elske" actually used in Norwegian for something other than human relationship? Or should it be "like" in use instead here?
Yes, it can.
We're almost more careful about how we use it for humans, as it then can make the distinction between platonic love/like for another person and romantic feelings, or the feelings a mother has for her child.
We do tend to reserve 'elsker' for stronger feelings of love/like, using it more sparingly than an American would do 'love', but this is not specific to animals.
This distinction is also becoming less and less clear among the younger generation - who are strongly influenced by the English language. They 'love/elsker' their new dress or phone just like an American teenage girl would, while this love for objects sounds a bit off to the older generations. All generations can use 'elsker/love' for food and actions as a stronger version of 'liker/like/enjoy'. "I love cake/swimming!", etc.
Seeing as fish can't live without water, using 'elsker' in this sentence sounds perfectly fine to my native ear. I would say that my cat 'elsker/loves' raw salmon or chicken meat, because he goes absolutely crazy over it, but that he 'liker/likes' his regular cat food. :)
Is this a statement about a specific group of fish? Or is this a generalization about all fish? That is, does it mean "All fish love water"? Or would that be "Alle fisker elsker vann"?