Ball of glowing light slowly circulating the water before immediately submerging.

As we began the descent from Bristol Airport (UK) to Dublin Airport (Ireland), the plane turned left, and there was a glowing orb above the water. The orb was approximately 10ft above the water and was slowly circulating. After 15 seconds, the orb instantly submerged into the water (a quick 90-degree turn downwards). It didn't slow in speed; it just shot down quickly. The ball/orb was consistent in its movements, circling in a perfect circle.
I can't say how large the object was, but it was large enough to be viewed from an airplane during its descent (we weren't majorly high at this period either). The object was a circular white object with a warm yellowish glow.
I've run some obvious explanations through my head, and it wasn't a reflection from something inside the plane. The flight was almost empty on the flight back, so I experimented with light reflections in the window. The reflective lights didn't have the same brightness and depth as the orb. There wasn't any double/triple light that you'd get from triple-barrelled airplane windows. On the same return flight, some lights from outside the airplane reflected off the engine. That type of light wasn't what I saw above the water, nor would lights that small reflect off the water so brightly that far down. It didn't look like a regular light from the distance, how that would look. It looked more like a circle that had a slight glow orbiting it.
This sighting occurred in Dublin Bay, the bay of sea close to the shore over the Dublin airport flight path. Between 7:15am and 7:30am. As I was in the middle seat next to a sleeping passenger, I didn't take any photos. The timeframe of me seeing the object and its submerging was around 10/15 seconds, so by the time I had my phone out and ready, it was gone.