I found the harbor nice but not too terribly interesting standing on the shore in Paihia. Same when I stood on the shore in Russell. You see most of the surrounding natural beauty when you are on a boat between Paihia and Russell. There is a fair amount of activity around the harbor which mostly consists of dolphin watching boats heading out to sea and the ferries running between the two parts of the bay. But as with all harbors the boats coming and going can be mesmerizing.
It is quite nice to walk along either shore and enjoy the views. Particularly on the Paihia side, you can walk from the ferry terminal all the way to the Treaty Grounds. This will take you an hour or so but the views are really nice.