Don't know about the hippos, but a lot of aggressive non-predators you're advised to act more like this, nonplussed and not scared. Like I've seen a video of a guy getting charged by a gorilla over and over and it stops inches away from him because its trying to intimidate him, but if he panicked and ran it might have gone further but the gorilla seemed to gauge his complete disinterest in attacking or disputing territory by his appearing disinterested
Though i wouldn't try it i would shit my pants and the gorilla would smell my shit and eat me