[Gaea to UE] Adding the Sea to the Landscape

Hello
I made an island in Gaia after watching this YouTube video and called it Unreal.
And I painted it with landscape paint.
But I couldn’t copy it perfectly.

As I try to add an ocean, there is a clear boundary between the island and the water as shown in the picture.
And the other islands are flattened.

So I want to ask.

First, is there a way to fill only that part with water using the black and white map of the picture?

Second, is there a way to clear the border between the island and the water?