Genuinely interesting one this. Creek sightings are underrated compared to lake and river reports but they can be more compelling because the animal has less room to manoeuvre and you're often closer to it.
Few questions - what time roughly, and was there any moonlight or were you using a torch? Also did it move more like something that swims low in the water or was it bulkier, like it was wading? That distinction matters a lot for narrowing it down.
In my experience the misidentification rate goes way up at night obviously, but people also tend to discount what they saw afterwards and that's the bigger problem. If your gut said it wasn't a beaver or otter then trust that, you know your local wildlife better than anyone.
Where are you based roughly? That'd help massively. Some out-of-place species reports cluster in specific regions and it'd be good to know if this fits a pattern anyone else here has flagged.