Right so I'm not usually one for mystery animals but this has got me proper intrigued. What time of night are we talking and how big is "huge" exactly - like labrador huge or something properly worrying?
I ask because a mate of mine near the Wash swore he saw something similar a few years back and everyone told him it was an otter, which is obviously ridiculous given what he described. Lincolnshire's got some weird water based sightings if you dig around, more than people realise.
Any chance you caught it on a trail cam or even just a phone video? Even blurry footage helps narrow things down. Also worth checking if it left any prints in the mud along the bank, that stuff doesn't lie.