How does Grenville's writing make this such a memorable and significant moment in the novel?

The fact that William has got himself a stable job where the likes doing what he does for a living is a memorable time for william because :

1)He is now able to live with Sal all the time.

2)He is glad to have got a job as a waterman because earlier collecting dog poo for the tanneries and wortink at the factories at such a tender age seemed dangerous and william did not like it.

Grenville also makes this moment such a memorable on because this job as a waterman with Mr. Middleton defines williams choices after he is deproted to New South Wales and this is shown when he choses to borrow a loan, persuasded by Sal to buy a boat, which later(after moving into the hawkesbury), he uses to transport goods to and back to Sydney, in the process earn a living.

I hope this makes complete sense and this fully anwers your question.

Thanks

Along with what has already been said, this moment is significant because it is the turning point in William's life. At this point he first starts to see the light at the end of the tunnel, the ray of hope. Hope that he may finally be able to make a living for himself and won't have to go to bed cold and hungry every night. This event is what leads him to becoming a 'man' from a boy, the fender for his family. Additional to that, he finally learns how life is like not being poor and having to scrape a meal every day. How to make a living without theft and with honesty. It is the moment that allows him to escape the poverty of his life and fulfill his dream of becoming a wealthy man. This is why this scene is so significant in Wiliam's life.

Hope this makes sense and helped to develop the answer previous asnwer. Thanks :)