Gets a face full of water and it stings its nose. Looses footing as it gets deep and doesn't like it. Hears the distant waves roaring and doesn't like it. Sees its human breath differently and move differently and doesn't like it. Looks at its owner and sees them getting small (just a bobbing head on the water) and doesn't like it.
Is that something people think? As far as I remember, I never felt unsafe swimming in any large body of water, give the conditions are alright. Based on my own experience, I would assume people default to thinking bodies of water are not dangerous, but probably depends a lot on the location of your upbringing (which for me was on an island).
Can be so many things.