John Newton had a strong sense of God's grace working in his life. He had an extraordinary number of close shaves, was whipped at one one stage as a deserter, was a slave trader and profane. That was why he described himself as a "wretch" in his hymn, "Amazing Grace".
ukwells.org
Yet somehow God got hold of him, and he ended up writing what is possibly the best known hymn in the English speaking world.
He also became a spiritual mentor to William Wilberforce in his successful attempt to get slavery outlawed.
It seems to me there was more a lot more at work than just his own choices. I believe he couldn't even swim.
Predestination is a mystery, and one that I struggle with. I've often said that the night my father died, he appeared in my room. At one point he blurted out "I always was doomed! I didn't really have any choice!" I argued back saying "That can't be right!" even though I was an atheist at the time.
But he replied "Oh, it's right, all right. You can see that from here!"
A little later he admitted "I was WILLING!" (to continue acting the way he did), so our "free" wills come into it.
John Newton still had to make a choice as to whether he would continue to follow God or not. But some of his shipmates didn't seem to be given the same opportunity and were drowned.