Mate, the lakes and reservoirs around this country have got some proper weird history attached to them, more than people realise. Earth mysteries researchers have been banging on about water as a kind of "hotspot" for unusual activity for years and honestly the evidence keeps stacking up.
What did it look like? Size, colour, how long was it there? The details matter more than people think when you're trying to work out what you actually saw versus what your brain decided it was afterwards.
I covered local news for years and the number of people who quietly pulled me aside to say "off the record, I saw something over the water" was genuinely surprising. Nobody wants to be the nutter so they just... don't report it. You're doing the right thing by getting it down while its fresh in your memory.