Those who are interested in everything nautical will want to visit Whangaroa Harbour. It's a nice place to go for a walk along the water's edge and admire some boats.
This place for ships to anchor in Northland comes recommended by travelers, so put it on your to-do list. However, don't end your sightseeing there. There are loads more options ready for you to discover after Whangaroa Harbour. Tauranga Bay, and Taupo Bay are not far away.