The Hippo-Sub is one of Aviva's various creature-inspired inventions. Designed and modeled after the hippopotamus, she had originally built it so as to fend off against crocodiles, when she saw a group of territorial hippopotamus go up against Crocodilla, a Nile Crocodile. It is generally used as a means of underwater transportation and exploration, secondary to the Amphi-Sub. It was introduced in the first episode Mom of a Croc.
In Bad Hair Day, the twin male lions found there way into the Tortuga, so Aviva, Koki, and Jimmy moved the lion cubs down to the Tortuga's garage, there they hid in the Hippo-Sub when the twins found the way down.
In Race for the Hippo Disk, the Kratt bros dropped the Hippo Power disk into a hippo filled river. Upset, Aviva announces they'll be taking the sub to receive it, so she and Martin took it down while Chris was on the lookout for Tusker (the bull hippo of the herd). When they found the disk they attempted to retrieve it, but got caught up in a fight between two male hippos and had to retreat.