Thanks for the contributions guys, and it seems as if there are almost a dozen zoos throughout Europe with underwater viewing for Common Hippos. I know that there also approximately a dozen zoos in North America with the same viewing option.
Calgary and Granby in Canada, plus at least 11 American zoos with underwater viewing for Common Hippos: Saint Louis, San Diego, Toledo, Sedgwick County, Disney's Animal Kingdom, Busch Gardens, San Antonio, Ellen Trout, Adventure Aquarium, Memphis and Dallas.
I've personally visited 10 out of the 13 exhibits, only missing Granby Zoo and then a couple of really new exhibits in Memphis and Dallas. My favourite would perhaps be the underwater viewing at San Diego Zoo, an establishment that also showcases Pygmy Hippos via underwater viewing!
