I downloaded the UAE map and installed on my garmin fenix5, after recording a swim activity, the plot on map shows my location wrongly, The log is supposed to show my track on the water but it shows I was on the land. There is I believe around 50meters difference, see the image below.
According to the picture that you posted, that map is provided by Google. It’s possible that they have the boundary between the sea and the land in the wrong place. This forum is for OpenStreetMap rather than Google Maps, though.
Another possibility is that the GPS in your Garmin Fenix watch is recording values 50m out (that’s not impossible if it’s not taking into account WAAS/EGNOS). That’s a question that you’d need to ask in Garmin’s support forums, though.
One thing that you might do is look at the maps at https://www.openstreetmap.org/#map=10/24.4195/54.3514&layers=C , zoom in to where you were swimming, and see if that is “in the sea” according to OpenStreetMap data.