Stu - may I suggest:
Heavy cavalry - Pygmy hippo
Medium/heavy - Warthogs
Medium cavalry - Capivara
Light cavalry - Deer or antelope of some description. I remember some
antelope in Zambia specific to swamps...Sitatunga.
Also, if you want really aquatic, then terrapins or boto (river dolphins who
have the added advantage of being seducers of maidens and so the fathers of
many an otherwise unexplained pregnancy)