We bought tickets online for the ferry from Hong Kong to Macau. Departure is at 12.45 tomorrow.
When should we arrive to the port? Is it like in the airport where registration closes 40 - 60 minutes before departure?
Fifteen to twenty minutes is generally enough. There's no registration to speak of. The longest part is passing through immigration, and, depending on the gate location, walking to the gate.
If it's your first time, I'd say arrive around 12:10. It'll be plenty.