Need to turn obstacle avoidance off, as it leans forward when you're close to the surface and it's moving I'd say your on point with a reflection tripping the sensor.I was flying above the sea but not as low as you and then the Mavic stopped ! Like there was an obstacle. Should be due to a reflect in the water I presume
The only real issue is wind, a gust can plough it down. I find land harder if it's not dead flat.Gutt renching flying that low. YOU WIN!
Good to know information.
Prop wash could also be an issue.