Looking for a little advice from the artists…

[Malc]

The coast where the blending between sea and land, circled in red, has a nasty black edge to it. Does this mean I need to do something with the Alpha channel, perhaps feather it on the land a little? Any advice would be appreciated - it took me ages to do this cos I am no artist myself…!!

[Arty]

This must be from the source photo like from shadow from the sun…
You could select the area (create a mask) by hand or by color and play with brightness and contrast. Another way is replace colors so from another or correct area duplicate the coloring in this area.
Final is use the clone tool duplicating near by area that is ok and play with transparency with the water so that it fades inside water.

[Malc]

Thanks Arty, you were correct - artifacts from the original satellite image, since I cut a cleaner line it is much better.