Can’t see it from that far out, but if you get really close your see there is no actual water covering the edges that’s why you get that edging because the water isn’t covering the ground below the see fully.
If you exclude the water further out from the coastline your see that the ground is green looking this is what is giving the shadow effect you mentioned.
Have fixed lots of this issue around the coast of japan with an add-on you just have to add water over those area’s where water is missing.
You will see in some places the opposite and the water is covering terrain that should be uncovered.
example here before and after:
