Some people show no courtesy! I have been thinking about this one, I know thw watres reasonably well.
1/ Jet Ski (quite a few round there)
2/ Someone leaving/picking up a mooring with a low slung transom/swim platform
or
3/ someone in quite a small boat (inflatable?) trying to fend off in the dark

That's all I can think of with a hole so low.
1 & 3 possibly returning from a trip to a local watering hole !!