72 hour visa Hong Kong

Sam, there is no 72 hour free transit in HK because HK and mainland PRC carry out the different exit and entry policy. But don’t worry, you can enjoy the free entry to HK for a stay of not exceeding 90 days. So no entry permit is needed.
As per what you said, you are not eligible for the 72 hour free transit in Guangzhou because the 72 hour policy is only applicable to flight travelers. However, if your layover in Guangzhou is less than 24 hours and after staying in Guangzhou, you will fly to Europe, you can enjoy the 24 hour direct transit there. In this case, no entry permit is needed either.

You need a visa to enter China mainland. And it takes about 3 hours from Hong Kong to Guangzhou by train or by bus. If you go to ferry, I am afraid it will be quite a long time. I come from Guangzhou. You can ask me if you need any help.