This is probably a matter of definition. As opposed to other beaches this sandy area seems to be regularly flooded *in its entirety* during high tides, which might disqualify it from being a proper beach per se. The other possibility that comes to mind is that the official boundaries of the harbour follow the waterline as currently drawn. My suggestion would be either:

* remove the sandy area if the harbour's boundaries need to remain as-is for formal correctness
* withdraw the water boundaries to those of the sandy area in the other case; although that will still carry ambiguity by marking a space as a beach that regularly is completely covered by water.

The main reason for this catch 22 is that there is no semantic notion for tidal zones in OSM's language definition I believe; please correct me if I'm wrong!