issuance of visa at JiuZhou port

You are perfectly allowed to do everything you have mentioned. You will be able to apply and obtain your Zhuhai VOA at Jiuzhou port, but keep in mind that there are only 4 daily ferries between HKIA and Jiuzhou and vice versa. In other words, you have to plan your entire trip well.

Fantastic news! Thank you so much for your information!
May I ask something else please?
1) Is the immigration office at JiuZhou port open 24/7 or I should also find the opening hours for it to plan my trip?
2) Should I bring with me an invitation letter from the people who invited me to go to Zhuhai or should I just say that I travel for leisure/shopping?

Thank you for your quick response by the way!

1) If you use any of the mentioned four ferries departing from HKIA, you will be able to obtain Zhuhai VOA at Jiuzhou port. Anyway, I seriously doubt that Jiuzhou port is open 24/7. This is something that I really cannot confirm.

2) No, you don't need any kind of invitation letter. It is better not to complicate the situation when your citizenship is perfectly eligible for this VOA. Anyway, having in mind that you intend to stay in Gongbei hotel, it is highly advisable to have that confirmation in hard copy. Also, leisure, shopping or similar is always the best phrase to be inserted in the application form.