Right so this is exactly the kind of thing I live for on this forum. Creek sightings are massively underreported imo, everyone fixates on forests and fields but water sources are where its at.
What did the movement look like? Was it slow and deliberate or more like something panicking and splashing through? That distinction matters a lot. A deer in water moves pretty awkwardly and makes loads of noise, so if it was quiet and smooth that rules out most common animals pretty quick.
Dorset here and we've had a few big cat reports near the Stour over the years which people just laugh off but there's more to it than the papers let on. Not saying that's what you saw but don't let people talk you out of reporting it properly.
Get down there in daylight and look for tracks in the mud before the ground dries out or it rains. That evidence disappears fast. Any chance you caught even a blurry photo or video on your phone?