According to Marlow, what for sailors is “the unpardonable sin”? How does this detail lend tension to the journey upriver?

The unpardonable sin is to scrape the bottom of your boat. This causes tension during Marlow's journey into the Congo because it's difficult to not occasionally scrape the bottom as they navigate the river.
The places they travel are not the wide shipping lanes of the ocean. Instead, they are winding paths deep in the jungle that make sailing the ship difficult. Marlow speaks of hidden channels and sunken stones that could hurt the ship and put everyone's lives at risk. Each hour they sail, the voyagers risk an accident that could be fatal for all of them.
Marlow says:

Imagine a blindfolded man set to drive a van over a bad road. I sweated and shivered over that business considerably, I can tell you. After all, for a seaman, to scrape the bottom of the thing that's supposed to float all the time under his care is the unpardonable sin. No one may know of it, but you never forget the thump—eh? A blow on the very heart. You remember it, you dream of it, you wake up at night and think of it—years after—and go hot and cold all over.

His statement exposes the anxiety that comes from the fear of hitting or damaging the bottom of the boat. It is a constant concern on their journey.