Ocean water appearing within elevated landscape (5-00)

I have a landscape generated from a heightmap, based on a coast, so at one side is the beach, on the other side is mountains. Therefore at the edge of my map, the mountains end in an elevated state, it’s not an island with beaches all the way around the edge.

When I add an oceanwater, i plot out where the edges of my landscape are, and plot a rough shape around the edge where the mountains are (as this side will never be seen)

But my water is rising up underneath my map and poking out where the mountains are, looking like little lakes - its like the splines I have added are creeping into my landscape and dont know where to make the shoreline.

If anyone has any idea why this is happening, I would love to know how to fix this.

Even when I have added more to my landscape to make it more island-like, the water still comes up through the landscape - BUT, it only happens where the mountains are. I assume its some kind of conflict between the landscape and the water.

Sorry about the incorrect tags, cannot seem to post if I use the UE5 tags