What does the following mean: "breaking top of an ocean roller"?

In My Humble Opinion
1

I’m looking for definitions for the terms “breaking top” and “ocean roller”.

“The landing on Mykines was exceptionally tricky. The boat surfed in on the green breaking top of an ocean roller into the tiny harbor …” (p.31 of Simon Winchester’s “Atlantic A Vast Ocean of a Million Stories”)

2

“Breaking top” is the breaking top of a wave. An “ocean roller” is a large wave.

In other words, to enter the harbour, the boats have to essentially “surf” in on large breaking waves.

3

Thanks Princhester.