Right so first thing I'd want to know is whether you noticed any displacement effect on the water surface directly beneath it. A lot of people skip over that detail but it's actually one of the more useful data points for ruling out things like drones or chinese lanterns.
Also what was teh quality of light coming from it - was it self-illuminating or did it look more like it was reflecting something? And did it hold a completely fixed position or was there any micro-drift? Hovering things that look totally static against wind conditions are genuinely interesting, anything with even slight movement usually has a boring explanation.
Whitby's right on the coast so I get plenty of weird stuff over the water near me and nine times out of ten its atmospheric or a drone some numpty is flying at 11pm. But occasionally something comes along that doesn't fit neatly into any of those boxes and those are the ones worth documenting properly.
Write down everything you remember now before the details start blurring. What time, duration, weather conditions, any sound at all.