I don't know if there's a scientific answer to this question. I think our eyes are not used to seeing with water constantly rushing in and out of our eyesockets so it's hard to see (almost like when someone first starts wearing contacts; it's blurry at first, but after getting used to them, they're fine and you can see fine). I think if we lived in water, our eyesight would get used to "living in water."
Hope that helps.