The answer would be the terrible stench. Before he was purchased as a slave by Col. Pascal, Equiano spent a lot of time below deck because he was said to be always sick on the voyage. Equiano finds it the place very revolting because of the foul odor from leftover food and human waste that was spread out over the place. They have limited mobility there because they were all chained up. There was also no place for them to do their business.
What did Equiano find most repellent about the conditions below-decks? the lack of light the terrible stench the horrible food the inability to communicate