Ptters are good for getting stuff in the water like pearls, black Pearl's and other stuff sometimes. They are also good for holding artifacts, and for keeping you warm in the snow biome or in the cold.
The otter has two abilities, one giving you more insulation and it scales with the otter's melee (so only level the melee) and it is also the only creature that can carry multiple artifacts at once. And it can also where a helmet.
They can carry multiple artifacts, even on official, and provide hypothermic insulation against the cold. Literally a must have for running artifacts if you plan to spam run boss fights for element. 10/10 would smash again.