How do you get fiber to the top of a mountain or the middle of the ocean?

That isn't a big problem due to several factors. To begin with, OFCs on the ocean bed is how route the majority of the internet traffic already. You can easily bury it a few inches under the surface anywhere. Another point is that these are often used only just high capacity backhaul links. You don't need to criss-cross the terrain with it. And even in the case of the worst of terrains, you can switch to high capacity, highly directional RF or Laser data links for short stretches.

Most people don't live on the top of a mountain or on boats.

So in that case a small satellite constellation is fine.