I love this idea, a LOT. I appreciate greatly you didn't make it OP like a lot of people do. It's fun to have strong dinos, but it gets boring when they're all so powerful with a million different abilities. They're just too strong and that's no fun.
I don't like the fins too much, but beyond that I like the design a lot. It's cute. The taming method is also very unique and exciting. Knock-out and passive taming are great, but I like a variety of passive taming. There are some creatures I think are better suited to passive tames, tbh... Jerboa (especially since it's so easy to kill them), dodo (because what's the point of having to punch a small bird but not the lystro?!) and others I can't think of atm.
I hope this gets added. I look forward to riding a giant otter.
Some ideas I've got to further add on:
1. Faster in the water than on land, like the otter. It's got short legs and a long draggy tail, after all. So it could have a speed to rival sarcos or other faster water tames in the water, BUT to counterbalance the water speed, it's slower on land. Maybe it's walking speed is similar to a dodo's run, and the running speed is similar to a stego's run? Swimming both regular and "sprinting" is not only faster, but it uses no stamina like the quadrupedal iguanodon. It's designed perfectly for water, so it can go through it easy peezy. Walking on land, however, DOES use stamina as it's more difficult for the giant otter.
2. You (or a comment) mentioned a saddle. I think it's silly that wolves don't get a saddle (not even optional!!!) but saberteeth need one. Maybe the otter can have an optional one? The armor isn't great as it would provide a large drag, especially if used to carry oxygen to extend your underwater visit. It can trade extended oxygen for slower travel OR little armor protection to make things even.