Rats are very intelligent animals, they have no fear of water and in fact are great swimmers, they are notorious for stowing away on ships, and are found in abundance near any source of water.
All we know about the Aumaua is that they are large and they live near the water.
Imagine a giant bi-ped humanoid river rat, long tail, hunched posture, able to jump long distances, fast reflexes, doesn't fear anything, and highly intelligent.
For reference think of the movie: The Secret of Nym.
Also think about the character Reepicheep from the C. S. Lewis book "Voyage of the Dawntreader".
A large bi-ped humanoid (Player Character) Rat race has never been done before in any game of which I am aware.
(EDIT: Just to clarify, I know that rat-folk exist in D&D and a lot of other settings, but as far as I am aware I've never seen a game where rat-folk were used as a PLAYER CHARACTER race. But of course, I could be wrong about that.)
I think this would lead to some very interesting romance threads.
<3