Late on this post but was finding the same issue…
I’m very neeeew to unreal and probably did some stupid solution but works for me, this time.
I have created a second ocean and scaled it down to the center of my island.
Then under terrain I unchecked the box “Affect Landscape”
That filled all holes of my island that was created with the first ocean…
don’t know if it’s clear.
And if you have found a better way to do it I’d love to hear 
