I think the bus stop is not far from the port entrance. It is a main street and you can see the buses coming. Look for a bus sign and people waiting for the bus. There are designated stops along the route. You ask the driver to announce your stop. If you are going to the Marina then ask the driver to tell you when they get there. Might be quote the hike to get to where you want at the Marina. It is a big area.
Might be better to take a taxi to be sure you get to the right place. The marina is confusing. There are a couple of places called the Marina. There is one where the tour boats leave and another where there are stores and restaurants. I think it would be better to get a taxi or Uber to go there. Much simpler if you are going to centro and the Malecon to take a bus