These wonderful creatures can be seen - for free - at the Horniman Museum's aquarium. It's in the basement of the museum, just a few minutes' walk from Forest Hill station on the East London Line.
If you fancy something a little less fishy, then don't miss the Horniman's famous walrus. He was brought from Hudson Bay, Canada in about 1870; he's a little overstuffed because the Victorian taxidermist didn't know where skin folds should be, and a little patchy because he's been on display for a very long time. However, like all the best stuffed animals, he's perhaps loved all the more for his imperfections.