I've never had altitude stability issues over water with my
MM1 so far, including flying as close as 0.5m above the water surface.
Last week I was flying over a river (approx 5m above the surface) and noticed that the aircraft was slowly drifting downstream. I let it hover and drift for a bit and it moved about 6-8m off its original position without any input from me. Most likely the VPS was active and it was locking on to ripples and eddies in the flow of the river, and prioritizing this over GPS. It drifted further than I expected for it having a good GPS signal, but then consumer grade GPS is actually a lot less accurate than we tend to think (5-10m accuracy is quite typical).
I've had a Tello lock on to and "chase" leaves that were being blown across the ground, but then the Tello doesn't have GPS.