What Equiano means by this is that he was so ill that the slavers thought that they would need to have him up on deck.  They felt that if he were not kept up on deck, he would die.  They also, Equiano says, felt that he was not really a danger because he was so young.  Therefore, they didn’t worry about having him up on deck and they did not even bother to chain him while he was up on deck.  Equiano uses the phrase “happily perhaps for myself” because he realizes that he was lucky to be brought up on deck where he would have a better chance of survival.

In the lines before the line you quote, Equiano gives a very vivid description of why being below decks in the hold would be a bad thing.  He talks about how hot it was and how closely the slaves were packed together.  He talks about the horrible smells from what he calls the “necessary tubs.”  Because of these conditions, many of the slaves who were in the hold became ill and many of them died.  This is why Equiano himself was so ill that he had to be brought up on deck.  He was lucky that he was young enough that the slavers felt it was safe to do so.