Right, so where exactly are you located? That's going to make a big difference to what it could be. Something massive in a creek narrows it down quite a bit depending on the country, let alone the region.
I've had a few odd encounters on Dartmoor over the years and the first thing I always ask myself is - could it be a known animal behaving unusually, or is the size genuinely incompatible with anything native? Because people massively underestimate how big an otter can look in low light, especially in moving water. Not saying thats what you saw, just worth ruling out.
What struck you most about it - the size, the movement, the shape? All three together is usually what separates a genuine mystery from a misidentification. Photos would be great obviously but even a rough sketch or description of the body proportions would help the thread a lot.