The sentence has a logical problem, that's why it looks wrong to you.

Columbus gazed at the land he had come/travelled so far to seek: he (had seen) nothing but ocean for five long weeks.

Past perfect describes something that happened in the past before something else.

Prior before seeing the land he sought he had seen nothing but ocean.

1-Columbus gazed out at the land he saw before him, 2- he had previously seen nothing but ocean.

Textbooks can complicate sentences. Often a simple rewrite into daily speaking sentences clarifies things.

1-What did columbus see 2-after sailing for so long 1-he saw land.

1-Columbus gazed out at the land he saw 2- he had previously seen nothing but land.

"Columbus gazed out at the land he saw before him, he had previously seen nothing but ocean".

In English the past perfect is usually used when something happens earlier than something else in the past. Both things happen in the past but one of them happens earlier than the second thing. In this case, Columbus had seen the ocean for five weeks before he arrived at the land he wanted to see, so that's why the past perfect is used. In other words, seeing the ocean for five weeks happened before he arrived at the land.