According to JapanDict, this proverb literally means "if a fish is friendly toward water, water will be kind to the fish too" (although Wiktionary says it's the other way around...). Is that really a literal translation? If it is, then where does this idea of friendliness come from? Do you get "a caring fish" by putting 魚 and 心 together? Can you do the same with other living creatures? Or does 魚心 only make sense when we look at the full phrase? Is it perhaps something like "when there is a fish with a (kind) heart, there is also water with a (kind) heart"?
Whatever the literal meaning is, is it obvious to a native speaker when they hear the proverb for the first time? Or do they need some mental effort to figure out how 心 works here?
Then there is stuff like 乙女心... Does it have any relation to this?