Especially since they're usually only on dry land at night while feeding.
Often during the day hippos sometimes move into little karungas that are heavily forested in order to escape the heat. The pools of water in these places are usually small, and not large enough to give the hippos the sense of security they have in a lake or river. Going into these places after them can be pretty exciting. When they are heading back to the river after a night's feeding, they can take exception to your presence and again can cause some drama.
My wife was scared to death of these things, so naturally became a hippo magnet. On our way back to camp one evening in the dark, a hippo calf ran along side the safari car close enough she could feel its breath on her hand and leg!
My wife being subjected to hippo desensitization, the trackers threw sticks at the hippos to prove to her that in deep pools they felt secure and wouldn't come after you . . . she wasn't a happy girl . . .
Their gaze could be intense though . . .
We ran into this guy just after leaving camp one morning
Our resident hippo pod, across the river from our camp, they are relaxed even though we were standing right at the water's edge . . . there was a crocodile around there somewhere too, although not in that pic
A lone bull near our side of the river . . .
In a small pool these guys were relatively exposed, and probably felt vulnerable so they quickly became uncomfortable with our presence, you can see that the guy in front has expanded his body to make himself appear as big as possible, that is often a sign that a charge is imminent . . .
We did find a hippo that wasn't dangerous, probably killed by lions as the tusks were still in the skull . . .
Despite the hippo drama, I tend to think that the lion is potentially the most fearsome animal in Africa, the hippo above might agree . . .