The concept is not that fish CAN'T recognize water, it's that it's more difficult for (metaphorical) fish that have always been immersed in water to notice wate

The concept is not that fish CAN'T recognize water, it's that it's more difficult for (metaphorical) fish that have always been immersed in water to notice water than for (metaphorical) animals that have seen both water and no water situations. The way it's usually stated is that fish are likely to be the last ones to notice water. It's the idea that you are more likely to ask yourself why traffic lights use red for stop and green for go instead of, say, the reverse the first day you visit a country that uses different colors than you were the previous day.