I love this so much. I've never known how badly I needed to be drowned by an otter before seeing this. My favourite kind of creature is one that can be both cute and terrifying and this sounds like it'd fit that so perfectly.
Feature recommendation I'd give is if you're underwater with the tamed one but not on top of them, they could use that same grab interaction that they would to drown you while wild to instead pull you to the surface to rescue you if your oxygen gets low!