After five shark attacks in six days in Egypt, a scientist believes a shark might have developed the taste for human blood. We’re screwed.
I was swimming with some friends in Florida a few years back and a lifeguard type drove up on an RV and yelled at us to get out of the water. We got all pissy and started giving him bluster about how there were no laws that said we couldn’t swim there, until he said, “there are sharks in the water.” That is a foolproof way to get me out of the ocean.
Tedquarters 
I think theres just something a little extra creepy about large animals in the water as opposed to land. I think it’s two fold.
One is the aquatic animal had a huge advantage. On land humans have a chance to get away, but in water humans just can’t swim well enough to outrun anything.
Second is that you can’t usually see it comming. If you are in chest deep water, even the clearest water, you likely won’t see something comming even if it’s right below the surface. I’ve swam and snorkelef with all types of things, even harmless stuff like stingrays, dolphins, giant sea turtles etc, and even though they are docile, you are always a little on edge in the water.