"In Sight of the landmark" just means exactly ONE or TWO spaces away. Nothing to do with vision. And since the ocean has no spaces....there you have it.
I think you've misunderstood the question. In the example the OP cites on page two of the rulebook, there are land hexes that are within two spaces of the palm tree, but wouldn't be considered to be "in sight of" the palm tree because they can only be "seen" across the ocean.