Right so before everyone jumps to "big fish" or "you were half asleep mate" - what exactly was the scale of it? Like relative to the width of the creek, how much of it did it take up? That's the detail that actually helps narrow things down.
Romania has had a few credible reports of massive catfish in river systems that people genuinely don't expect, some of these wels catfish get to sizes that would make anyone do a double take in low light. But there are also older accounts from rural areas here about things in waterways that don't really fit any known species, which is the part I find more interesting honestly.
What time was it, what was the light like, and did it make any sound or disturbance on the surface? The behaviour is usually more telling than the sighting itself. Also - did it seem aware of you? That one detail gets overlooked a lot but its actually pretty significant in terms of what category of encounter this might fall into.