I'll start
1. Sea otters have pouches in their fore arms. For their tools and some food for their babies.
2. The Hairy nosed otter is the most endangered species of otter population being around less than 100 last I checked.
3. Unfortunately out of the 13 species of otter we have right now, most of them are either declining in population or endangered.
4. Otters are usually looked for population status among scientist, it gives them information of both land and water (if either are polluted or not). For Sea otters they are 100% population control for urchins to keep the kelp around for marine life.
5. Sea otters are the heaviest otter.
6. Otters play some roles in mythology (which you should consider looking up).
In some Native American, there's a god that takes a form of one and is mischievous yet benevolent.
In Scottish Mythology the "Otter King" is a beast that is usually indicated by being surrounded by 7 black otters. If captured they can grant you 1 wish in return for their freedom. Though their pelt makes a warrior invincible, but the Otter Kings are hard to kill since their soft spot is around their chin.
7. Sea otters have the THICKEST fur of any animal.