The ferry terminal comes with one passenger terminal and can be upgraded with a CRUISE SHIP terminal, yet for me it says that limit is reached for an additional passenger terminal. Is this intended, that a cruise ship terminal is small enough to fit onto the dock but not a passenger terminal, or is there something I have to do?
If you want to expand the capacity of your ferry system, you'll need to build another dock entirely. Each one accommodates a ferry terminal and an optional cruise ship terminal.
This is similar in design to train stations. Although sims will have thought bubbles about expanding station capacity, the reality is that you have to make another station. There is currently no way to add additional rail bays to an existing station.