Hippos are vindictive as fuck, and highly territorial. They can spend a decent amount of time submerged, and swim better than people. Generally, you don't know you've wandered into Hippo town until it's too late. They will destroy, or at the very least capsize, small boats. I'd have to go make sure if it's still the case, but last time I checked they were classified as the most dangerous creature on the African continent barring Mosquitoes (Malaria kills more people).
Would the bear win? Maybe. What shape would he be in though? Hippos are 3 ton armored tanks with foot long canines applying 2,000 psi. Any wounds they inflict would be grievous.